Developed by Scott B Britton, GIA Graduate Gemologist, Gemology 101: Principles and Practices is a comprehensive PowerPoint presentation designed for jewelry professionals, students, and gem connoisseurs and collectors alike that explains in detail the basics of gemology and how to use the information to identify unknown gems.
Gemmology is a vast subject to learn about all the Gemstones, their properties, occurrences, geographical location, geometrical structure or formations, various colour shades, transparency to opaque, sheen & luster, cuts & shapes, carat & cost, testing through various instruments & devices for their Real, synthetics or simulants.
Introduction; Chemical composition of garnet; Structure; Classification; Physical properties; Optical properties; Occurrences; Gem variety; and Uses
Garnet group of minerals is one of the important group of minerals.
Since they are found in wide variety of colours, they are also used as gemstones.
Garnet group of minerals are also abrasives and thus have various industrial applications.
Gemmology is a vast subject to learn about all the Gemstones, their properties, occurrences, geographical location, geometrical structure or formations, various colour shades, transparency to opaque, sheen & luster, cuts & shapes, carat & cost, testing through various instruments & devices for their Real, synthetics or simulants.
Introduction; Chemical composition of garnet; Structure; Classification; Physical properties; Optical properties; Occurrences; Gem variety; and Uses
Garnet group of minerals is one of the important group of minerals.
Since they are found in wide variety of colours, they are also used as gemstones.
Garnet group of minerals are also abrasives and thus have various industrial applications.
Gold is a transitional metal. In its purest form have reddish yellow color, soft, malleable, and ductile metal.
Atomic number : 79
Atomic mass : 196.9 u
Density : 19.32 g/cm3
Melting point : 1,064 °C
Boiling point : 2,700 °C
Founded in different form associated with different rock type in different tectonic setting.
Discovered from earlier time and used for multi purposes.
Formation of gold
The saying among prospectors that "gold is where you find it" suggests its occurrence is unpredictable, but there is some certain geological environments for the formation.
Because gold is very stable over a range of conditions, it is very widespread in the earth’s crust.
Gold dissolved in warm to hot salty water, the fluids are generated in huge volumes deep in the Earth’s crust as water-bearing minerals dehydrate during metamorphism.
Any gold present in the rocks being heated and squeezed is sweated out and goes into solution as complex ions.
In this form, dissolved gold, along with other elements such as silicon, iron and sulphur, migrates wherever fractures in the rocks allow the fluids to pass.
The direction is generally upwards, to cooler regions at lower pressures nearer the Earth’s surface.
Gold eventually becomes insoluble and begins to crystallize, most often enveloped by quartz.
The association of gold and quartz vein forms one of the most common types of "primary gold deposits".
India
In India, gold mineralization of economic importance is mainly restricted to Archean greenstone terranes of the Dharwar Craton (DC).
The eastern block of the DC has a high favorability for hosting major gold deposits such as Kolar, Hutti, and Ramagiri, whereas the western block hosts only a few smaller deposits such as Gadag, Ajjahanahalli, and Kempinkote.
Gold also discoverrd by GSI in the Singbhum Craton, Aravalli Craton, Bastar Craton and Southern Granulite Terrain (SGT).
India is the second-largest consumer of gold after China.
India currently holds about 558 tones of gold, representing 6.6% of its reserves, (World Gold Council, October 2016).
Kolar Gold Field, Hutti Gold Field and Ramgiri Gold Field are the most important gold fields.
Gold Demand and Use
The largest source of demand is the jewelry industry Gold’s workability, unique beauty, and universal appeal make this rare precious metal the favorite of jewelers all over the world.
Besides jewelry, gold has many applications in a variety of industries including aerospace, medicine, dentistry, and electronics for the manufacture of computers, telephones, televisions...
The third source of gold demand is governments and central banks that buy gold to increase their official reserves.
Private investors there are private investors. Depending upon market circumstances, the investment component of demand can vary substantially from year to year.
A List Of Precious And Semi Precious Gemstones9gemeu
There are so many types of gemstones available today. We have created a comprehensive list of gemstones to guide you and help to find the perfect gemstone. others to be semi-precious, more are considered fine or ornamental. Differences apply. What stones are considered precious and which are those considered semi precious
GEOLOGICAL THERMOMETERS
DEFINITION AND CLASSIFICATION
Proper understanding of origin of mineral deposits and their classification requires the knowledge of formation-temperatures of these deposits. Certain minerals, present over there, give information’s with regard to temperatures of their formations and of the enclosing deposits and they are known as geological thermometers. These geological thermometers may be classed chiefly into the following groups based on their preciseness:
1. The thermometers that record fairly accurately the specific temperature condition of formation of deposits.
2. The thermometers that provide an upper or a lower temperature, above or below which the deposits do not form
3. The thermometers that provide a range of temperature within which the deposits form; and
4. The thermometers that serve as rough indications of temperatures of formation of mineral deposits.
The presence of two or more of less precise geological thermometers in a deposit narrows the range of temperature of formation for the deposits
To review the geology of rare earth elements.
To discuss the various applications of rare earth elements in geological studies.
To review the geology of rare earth elements.
To discuss the various applications of rare earth elements in geological studies.
Abundance of REEs in Earth’s Crust ; Classification of Rare Earth Elements; Geology of REEs; APPLICATIONS OF REES; Application OF REEs in Geological Studies; APPLICATION OF REE TO PETROLEUM SYSTEMS; REE GLOBAL ECONOMIC SUPPLY AND DEMAND; Large and Giant Sized Deposits of the Rare Earth Elements
Texture of Ore Minerals; Importance of Studying Textures; Individual Grains Properties; Filling of voids; Texture Types; Genetically differentiated between Texture types; Secondary textures from replacement; Hypogene Texture; Supergene Texture; Primary texture formed from Melts; Primary texture of open-space deposition; Secondary textures from cooling; Secondary textures from deformation; TEXTURES OF ECONOMIC ORE DEPOSITS; Textures of Magmatic ores; Cumulus textures; Intergranular or intercumulus textures; Exsolution textures; Textures of hydrothermal ore deposits and skarns; Replacement textures; Open space filling textures; Textures characteristic of surfacial or near surface environments and processes; Criteria for identifying replacement textures; Vein and Veining have different Nature Features
Gold is a transitional metal. In its purest form have reddish yellow color, soft, malleable, and ductile metal.
Atomic number : 79
Atomic mass : 196.9 u
Density : 19.32 g/cm3
Melting point : 1,064 °C
Boiling point : 2,700 °C
Founded in different form associated with different rock type in different tectonic setting.
Discovered from earlier time and used for multi purposes.
Formation of gold
The saying among prospectors that "gold is where you find it" suggests its occurrence is unpredictable, but there is some certain geological environments for the formation.
Because gold is very stable over a range of conditions, it is very widespread in the earth’s crust.
Gold dissolved in warm to hot salty water, the fluids are generated in huge volumes deep in the Earth’s crust as water-bearing minerals dehydrate during metamorphism.
Any gold present in the rocks being heated and squeezed is sweated out and goes into solution as complex ions.
In this form, dissolved gold, along with other elements such as silicon, iron and sulphur, migrates wherever fractures in the rocks allow the fluids to pass.
The direction is generally upwards, to cooler regions at lower pressures nearer the Earth’s surface.
Gold eventually becomes insoluble and begins to crystallize, most often enveloped by quartz.
The association of gold and quartz vein forms one of the most common types of "primary gold deposits".
India
In India, gold mineralization of economic importance is mainly restricted to Archean greenstone terranes of the Dharwar Craton (DC).
The eastern block of the DC has a high favorability for hosting major gold deposits such as Kolar, Hutti, and Ramagiri, whereas the western block hosts only a few smaller deposits such as Gadag, Ajjahanahalli, and Kempinkote.
Gold also discoverrd by GSI in the Singbhum Craton, Aravalli Craton, Bastar Craton and Southern Granulite Terrain (SGT).
India is the second-largest consumer of gold after China.
India currently holds about 558 tones of gold, representing 6.6% of its reserves, (World Gold Council, October 2016).
Kolar Gold Field, Hutti Gold Field and Ramgiri Gold Field are the most important gold fields.
Gold Demand and Use
The largest source of demand is the jewelry industry Gold’s workability, unique beauty, and universal appeal make this rare precious metal the favorite of jewelers all over the world.
Besides jewelry, gold has many applications in a variety of industries including aerospace, medicine, dentistry, and electronics for the manufacture of computers, telephones, televisions...
The third source of gold demand is governments and central banks that buy gold to increase their official reserves.
Private investors there are private investors. Depending upon market circumstances, the investment component of demand can vary substantially from year to year.
A List Of Precious And Semi Precious Gemstones9gemeu
There are so many types of gemstones available today. We have created a comprehensive list of gemstones to guide you and help to find the perfect gemstone. others to be semi-precious, more are considered fine or ornamental. Differences apply. What stones are considered precious and which are those considered semi precious
GEOLOGICAL THERMOMETERS
DEFINITION AND CLASSIFICATION
Proper understanding of origin of mineral deposits and their classification requires the knowledge of formation-temperatures of these deposits. Certain minerals, present over there, give information’s with regard to temperatures of their formations and of the enclosing deposits and they are known as geological thermometers. These geological thermometers may be classed chiefly into the following groups based on their preciseness:
1. The thermometers that record fairly accurately the specific temperature condition of formation of deposits.
2. The thermometers that provide an upper or a lower temperature, above or below which the deposits do not form
3. The thermometers that provide a range of temperature within which the deposits form; and
4. The thermometers that serve as rough indications of temperatures of formation of mineral deposits.
The presence of two or more of less precise geological thermometers in a deposit narrows the range of temperature of formation for the deposits
To review the geology of rare earth elements.
To discuss the various applications of rare earth elements in geological studies.
To review the geology of rare earth elements.
To discuss the various applications of rare earth elements in geological studies.
Abundance of REEs in Earth’s Crust ; Classification of Rare Earth Elements; Geology of REEs; APPLICATIONS OF REES; Application OF REEs in Geological Studies; APPLICATION OF REE TO PETROLEUM SYSTEMS; REE GLOBAL ECONOMIC SUPPLY AND DEMAND; Large and Giant Sized Deposits of the Rare Earth Elements
Texture of Ore Minerals; Importance of Studying Textures; Individual Grains Properties; Filling of voids; Texture Types; Genetically differentiated between Texture types; Secondary textures from replacement; Hypogene Texture; Supergene Texture; Primary texture formed from Melts; Primary texture of open-space deposition; Secondary textures from cooling; Secondary textures from deformation; TEXTURES OF ECONOMIC ORE DEPOSITS; Textures of Magmatic ores; Cumulus textures; Intergranular or intercumulus textures; Exsolution textures; Textures of hydrothermal ore deposits and skarns; Replacement textures; Open space filling textures; Textures characteristic of surfacial or near surface environments and processes; Criteria for identifying replacement textures; Vein and Veining have different Nature Features
Gemology is the scientific study of gemstones. Its practitioners, called gemologists, are experts in identifying gems and grading their purity, as well as authorities in the art of jewelry creation and stone setting.
Insights and tips on what to look for when shopping for and choosing the 'perfect' Birthday Gift or Engagement Ring. Did you know, several months have more than 1 'accepted' birthstone?
Tip: Gemstone jewelry makes the BEST Birthday Gift.
This presentation covers topics related to polarization of light, methods of producing polarised light, optical activity, birefringence etc.
This would help for quick revision of the topics.
Quartz is found in almost every geological environment. It is a common constituent in most of the rock types and soil groups. Granite, sandstone, limestone, and most of the igneous, sedimentary, and metamorphic rocks contain quartz. Quartz contains mainly oxygen and silicon. These two constituents make upto 75 % of the earth’s crust. An alternate name for the Quartz Group is the Silica Group.
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2. Class Outline
Brief History Of Modern Gemology
Basic Gemology Theory
Primary Gemology Tools
Secondary Gemology Tools
External Laboratory Tools
Putting It All Together
Hands-On Lab Time
3. Class Goals
Teach Basic Gemology Principles
Give You Experience With Primary
Gemology Tools And Gem Identification
Familiarize You With External Lab Tools And
When To Take Advantage Of External Labs
4. What The Class Will Not Cover
Diamond Grading
Colored Stone Grading
Precious Metals Testing
Any Material Outside The Realm Of Basic Gem
Identification
5. Disclaimer
Although The Information Contained In This
Presentation Is Derived From GIA Material, This
Class Does Not Substitute For Official GIA
Coursework. This Class Will Introduce You To
Basic Gem Identification, But Will Not Cover All
The Complexities Involved In Making Positive ID's
Of Unknown Gems.
7. Before Modern Gemology
Transparent Red Gems Were Considered
Rubies
Transparent Blue Gems Were Considered
Sapphires
Transparent Green Gems Were Considered
Emeralds
8. Before Modern Gemology
(Continued …)
There Are A Few Famous Examples Of Gems
Misnamed Because Of This Type Of
Misclassification. For Example, The Large Red
“Ruby” In The British Crown Jewels Is Actually A
Red Spinel
9. Before Modern Gemology
(Continued …)
If Someone Wanted To Examine Or Test A Gem,
The Only Available Methods Were Chemical
And/Or Physical And Would Require Either The
Gem Be Destroyed Or, At Minimum, Need
Recutting And/Or Repolishing (With Weight Loss)
10. Modern Gemology
Late Nineteenth Century To Present
Progression Pushed Forward By The Discovery
Of Diamonds In South Africa
Goal Was To Develop A Non-Destructive,
Scientific Method Of Classifying Gems
11. Modern Gemology
(Continued …)
In 1931 Robert M. Shipley Starts GIA, Whose
Mission Is To Ensure The Public's Trust Of
Diamonds, Colored Stones, And Pearls
Through Education And Research
Richard T. Liddicoat, Jr., The “Father Of
Modern Gemology,” Develops The Modern GIA
Diamond Grading Scale In 1953
13. Refraction Is The Principle That A Beam Of Light
Traveling In One Medium Will Bend And Either
Speed Up Or Slow Down As It Enters Another
Medium
Refraction Index
14. Refraction Index
(Continued …)
The Ratio Of The Velocity Of Light In The Old
Medium To The Velocity Of Light In The New
Medium Is Known As The Refractive Index.
16. Refractive Index
(Continued …)
Measured From The Ratio Of The Velocity Of
Light In Air To The Velocity Of Light Inside A
Gem, Refractive Index Is A Unique “Fingerprint”
For Each Gem
However, Refractive Index For Each Gem Type
Can Be Broken Down Even Further Depending
On The Crystal Structure Of The Gem
17. Movement Of Light
As Light Moves Through Air (Or Any Other
Gas), It Vibrates Perpendicularly In Waves
From The Direction It Was Transmitted.
When Light Enters Another Medium, Not Only
Does The Velocity Of The Light Change, The
Direction It Is Transmitted Is Affected By The
Crystal Structure Of The New Medium
19. Single Refraction
Single Refraction Is When A Gem's Structure
Does Not Change The Vibration Of The Light
Waves Transmitted Other Than Changing The
Velocity, Direction, And/Or Partially Absorbing
The Light Waves' Energy
Light Waves In A Gem That Has Single
Refraction Vibrate Evenly In All Directions
20. Single Refraction
(Continued …)
Gems That Fall Under The Category Of Being
Amorphous (Lacking A Regular Crystal
Structure) Or That Crystallize In The Cubic
Crystal System Are Singly Refractive
Singly Refractive Gems Have One Refractive
Index Ratio
21. Double Refraction
Double Refraction Is A Condition Where One
Light Wave Is Split Into Two Light Waves
Traveling Perpendicular To One Another And
Vibrating At Different Rates As It Enters The
New Medium From Air (Called Polarization)
24. Double Refraction
(Continued …)
Gems That Are Doubly Refractive Have
Refractive Indices That Vary Between A
Minimum And Maximum For Each Gem
Depending On The Direction Of Observation
The Measure Of A Gem's Ability To Convert A
Single Ray Of Light Into Two Unequal Velocity
Waves Is Known As Birefringence
25. Double Refraction
(Continued …)
Doubly Refractive Gems Can Be Further
Broken Down Into Two Subcategories: Uniaxial
And Biaxial
Uniaxial Gems Have One Direction, Called The
Optic Axis, Where They Do Not Polarize Light
Biaxial Gems Have Two Directions Where They
Do Not Polarize Light
26. Double Refraction
(Continued …)
Uniaxial Gems Can Be Positively Or Negatively
Signed
Uniaxial With Positive Sign Have A Fixed Top
R.I. And A Varying Bottom R.I. Over A 180º
Cycle. In Addition, Every 180º The Varying
Bottom R.I. Equals The Fixed Top R.I.
28. Double Refraction
(Continued …)
Gems That Are Uniaxial With Negative Sign Are
Similar To Their Positive Siblings With The
Exception That The Top R.I. Varies And The
Bottom R.I. Is Fixed Over A 180º Cycle. In
Addition, Every 180º The Varying Top R.I. Equals
The Fixed Bottom R.I.
30. Double Refraction
(Continued …)
Biaxial Gems Can Be Positively Signed,
Negatively Signed, Or Without Sign
Positively Signed Biaxial Gems Are Where
Both R.I.'s Vary, But The Bottom R.I. Varies
More Than The Top R.I Over A 180º Cycle. In
Addition, They Reach A Common
Measurement At Some Point In The 180º Cycle
32. Double Refraction
(Continued …)
Negatively Signed Biaxial Gems Are Similar To
Their Positively Signed Siblings With The
Exception That The Top R.I. Varies More Than
Bottom R.I. Over A 180º Cycle. In Addition, They
Reach A Common Measurement At Some Point
Over The 180º Cycle
34. Double Refraction
(Continued …)
Gems That Are Biaxial Without Sign Have Top
And Bottom R.I.'s That Vary Equally Over A 180º
Cycle. In Addition, The Top And Bottom R.I.'s
Reach A Common Measurement At Some Point
Over The 180º Cycle
37. Primary Gem Identification Tools
Dichroscope (Mid-To-Late Nineteenth Century)
Refractometer (1885)
Polariscope (First Developed For Gem Industry
In 1935)
Gemological Microscope (First Developed For
Gem Industry In 1938)
39. Dichroscopes
Dichroscopes Work On The Principle That Light
Waves That Are Perpendicular To One Another
And Vibrating At Different Frequencies Will
Display Different Colors
Dichroscopes Allow A User To Check A Gem
For Pleochroism, A Condition Where Different
Colors Show From Different Viewing Angles
40. Dichroscopes
(Continued …)
Dichroscopes Use A Clear Medium With High
Birefringence (Normally Calcite Or Polarized
Polaroid Filters) To Allow A User To View Two
Colors Per Direction In A Doubly Refractive Gem
(If Distinct, And Except In The Direction Of The
Optic Axis)
42. Dichroscopes
(Continued …)
Dichroscopes Can Only Be Used With Gems
That Are Transparent To Translucent
To Use A Dichroscope, You Hold The Unknown
Gem In Front Of A White Light And Then View
The Gem Through The Instrument
You Must View Gems From At Least Three
Different Directions To Check For Pleochroism
43. Dichroscopes
(Continued …)
Gems That Show The Same Two Colors In All
Directions Where Light Is Polarized Are
Dichroic
Gems That Show Three Colors (But Only Two
At A Time In Each Single Direction Where Light
Is Polarized) Are Trichroic
46. Refractometers
Probably The Best Tool For Gem Identification
Refractometers Are Used To Measure The
Refractive Index (Indices) Of An Unknown Gem
47. Refractometers
(Continued …)
Refractometers Work On The Principle Of
Measuring The Critical Angle (Bending) Of
Light Entering And Exiting The Gem From A
Known Medium (Usually A High Lead Glass
Surface)
48. Refractometers
(Continued …)
There Are Two Methods For Taking Refractive
Indices In A Gem: The Spot Method And The
Flat Facet Method
The Spot Method Is Used To Take R.I.'s When
There Is Not A Flat Facet On A Gem
49. Refractometers
(Continued …)
You Will Typically Get Only One R.I. Reading
Using The Spot Method Unless The Gem Has
High Birefringence
The R.I. Value Obtained From The Spot
Method Will Vary Between The Minimum And
Maximum R.I.'s For A Doubly Refractive Gem
Depending On The Direction Taken
50. Refractometers
(Continued …)
The Most Common Method For Reading A
Spot R.I. Is To Read The R.I. Value Where The
Gem Is Half Lit, Half Dark (Called The “50/50”
Method)
Sometimes The “50/50” Method Does Not Work
And You Must Find The Average Between
Where The Gem Is Lit And Where It Is Dark
52. Refractometers
(Continued …)
The Flat Facet Method Requires You To Take
R.I.'s In Seven Different Directions From 0º To
180º (Every 30º) On At Least One Facet Of A
Gem
You Take A R.I. Reading For A Gem Where The
Colored Or Shadowed Line Ends Using The Flat
Facet Method
54. Refractometers
(Continued …)
Turning A Polarized Filter 90º Over The
Refractometer's Viewing Lens Checks For The
Presence Of A Second R.I. Using The Flat Facet
Method
56. Refractometers
(Continued …)
Singly Refractive Gems Will Show The Same
Refractive Index (+/- Allowance For Each Gem
Species/Variety) For Any Direction That The
Refractive Index Is Taken Using Either The Spot
Method Or The Flat Facet Method
57. Refractometers
(Continued …)
Depending On The Angle, Direction, And Type Of
Doubly Refractive Gem, Either You Will Get Two
R.I.'s Between A Minimum And Maximum
Threshold (For Directions Where Light Is
Polarized) Or A Single R.I. (For Directions Where
Light Is Unpolarized) For R.I.'s Taken With The
Flat Facet Method
58. Refractometers
(Continued …)
If You Only Get A Single R.I. After You Have
Taken Measurements 180º On A Single Side Of A
Gem, You Should Take Measurements Again On
At Least Two More Sides Because Doubly
Refractive Gems Do Not Polarize Light In At Least
One Direction (The Optic Axis)
59. Refractometers
(Continued …)
To Find The Birefringence Of A Doubly Refractive
Gem, You Subtract The Lowest Refractive Index
Recorded On A Single Side Where Light Is
Polarized From The Highest Refractive Index
Recorded On The Same Side
60. Refractometers
(Continued …)
Although Refractometers Are The Best Tool For
Reading Refractive Indices, They Are Restricted
To Taking Refractive Index Values From 1.30 To
Either 1.81 Or The Refractive Index Of The Fluid
Used To Make The Make The Contact Between
The Refractometer's Hemicylinder And The Gem
61. Refractometers
(Continued …)
If You Record A Refractive Index Reading Of
1.81, In Most Cases You Will Need To Rely On
Other Tests To Identify The Unknown Gem
There Is An Instrument Called A Reflectivity
Meter Which Can Measure R.I.'s Above 1.81,
But It Is Less Reliable Than A Refractometer
62. Refractometers
(Continued …)
The Refractometer Serves As One Method For
Finding The Optic Character (Uniaxial Or
Biaxial) For Doubly Refractive Gems Using The
Flat Facet Method
63. Refractometers
(Continued …)
Using A Piece Of Graph Paper And The 14
High And Low Refractive Indices, Plot The
Values Horizontally Starting At 0º And Ending
At 180º
64. Refractometers
(Continued …)
If Your Graph Has An R.I. Line That Is Fixed And
One That Varies Without Meeting At A Common
Measurement Somewhere In The 180º Cycle Or
You Have Two Fixed Lines, Then You Have An
Uniaxial Gem, But You Will Need To Retake The
R.I.'s On A Different Gem Side To Determine Sign
66. Refractometers
(Continued …)
If Your Graph Has Two R.I. Lines That Vary But
Do Not Have A Common Measurement In The
180º Cycle, Then You Have A Biaxial Gem, But
You Will Need To Take R.I.'s On A Different Gem
Side To Determine Sign
69. Polariscopes
Useful For Detecting Whether A Gem Is Singly Or
Doubly Refractive (Or An Aggregate) And The
Optic Character (Uniaxial Or Biaxial) And
Pleochroism Colors (If You Do Not Have Access
To A Dichroscope) In Doubly Refractive Gems
70. Polariscopes
(Continued …)
Polariscopes Work On The Principle Of Placing A
Transparent To Translucent Gem Between
Crossed, Polarized Analyzing Windows And
Viewing The Reaction Of The Gem As Light Is
Passed Through The Bottom Analyzing Window,
Through The Gem, And Finally Exiting The Top
Analyzing Window To Your Eyes In A 360º Cycle
71. Polariscopes
(Continued …)
Crossing polarized analyzing windows causes
light being transmitted from the polariscope's
base through the two analyzing windows to be
blocked and the view to be dark
72. Polariscopes
(Continued …)
If You Place A Gem In Between Crossed
Analyzers And Light Is Not Visible As You Rotate
It 360º In At Least Three Different Directions, Then
The Gem Is Singly Refractive And You Are
Finished With Polariscope Testing
73. Polariscopes
(Continued …)
If The Gem Appears Visible As You Rotate It 360º
In At Least Three Different Directions Between
Crossed Analyzing Windows As Light Is Being
Transmitted, Then The Gem Is An Aggregate And
You Are Finished With Polariscope Testing
74. Polariscopes
(Continued …)
Gems That Alter The Visibility Of Themselves
In The Polariscope Are Either Singly Refractive
Showing Anomalous Double Refraction Or Are
Doubly Refractive
Gems That Are Singly Refractive Showing
A.D.R. Are The Result Of Irregularities Within
The Crystal Structure
75. Polariscopes
(Continued …)
Gems That Are Doubly Refractive Display Light
Every 180º Between Crossed Analyzing Windows
Because Light Is Polarized At 90º Angles Within
The Gem's Crystal Structure And Essentially
Cancels The Polarization Of Light Between The
Analyzing Windows
77. Polariscopes
(Continued …)
To Separate Whether A Gem Is Singly Refractive
Showing A.D.R. Or Doubly Refractive, Turn It To
The Brightest Lit Position And Then View The
Reaction Of The Gem As You Turn The Top
Analyzing Window So That The Two Analyzing
Windows Are No Longer Crossed And Are
Displaying Light
78. Polariscopes
(Continued …)
If The Gem Gets Lighter Than It Was Through
Crossed Analyzing Windows, Then It Is Singly
Refractive
If The Gem Stays The Same Or Gets Darker
Than It Was Through Crossed Analyzing
Windows, Then It Is Doubly Refractive
79. Polariscopes
(Continued …)
It Must Be Noted That The Polariscope Has One
Significant Limitation. Certain Red Gems With A
Refractive Index Above 1.71 Can And Do Give
False Readings When Viewed With The
Polariscope. Therefore, You Should Rely On
Other Tests To Confirm Single Or Double
Refraction For This Category Of Gems
80. Polariscopes
(Continued …)
You Can Determine The Optic Character
(Uniaxial Or Biaxial) Of A Doubly Refractive
Gem Using The Polariscope
You Turn The Analyzing Windows So That
They Are Crossed And Hold The Gem In
Between The Two Windows, Looking For A
Color Pattern Similar To An Oil Slick On Water
81. Polariscopes
(Continued …)
Using A Condensing Sphere And The
Refractometer's Magnifier, You View The
Colors Through The Condensing Sphere And
Note The Appearance
If The Colors Have A “Bow Tie” Appearance In
The Condensing Sphere, The Gem Is Biaxial
83. Polariscopes
(Continued …)
If The Appearance Of The Colors In The
Condensing Sphere Shows A Shape Similar To
An Iron Cross, A Bull's Eye, Or A Pinwheel
(Airy's Spiral), Then The Gem Is Uniaxial
You May Or May Not Be Able To Resolve The
Optic Character Using The Polariscope
86. Polariscopes
(Continued …)
You Can View Pleochroism Colors In A Doubly
Refractive Gem Through The Polariscope By
Crossing The Analyzing Windows And Viewing
The Gem Every 45º Where The Light Is Visible In
A Direction Where The Gem Polarizes Light
88. Good Gemological Microscopes Work On The
Principle Of Magnifying A Gem In Stereo (As If
You Were Looking At The Object Without
Magnification)
The Best Tool For Observation Of Inclusions,
Blemishes, Separating Synthetics, Simulants,
Or Imitations From Natural Gems, Etc.
Microscopes
89. Microscopes
(Continued …)
You Should Begin All Work Sessions First By
Adjusting The Magnification Of Your
Microscope Because Temperature, Movement,
And Atmosphere Affect The Focus Of The
Optics
You Begin Focusing The Microscope By
Adjusting The Width Of The Oculars
91. Microscopes
(Continued …)
You Then Take The Focusable Ocular Out,
Turn The Zoom To 10x, And Focus The
Remaining Stationary Ocular With The
Microscope Arm Focus Adjustment On The
Small Lit Hole In The Iris Diaphragm
92. Microscopes
(Continued …)
You Then Take Out The Nonfocusable Ocular,
Put In The Other Focusable Ocular, And Then
Adjust That Ocular At The Ocular Base Until
The Small Lit Hole Comes Into Focus
You Then Put The Other Ocular In And Check
To See If The Focus Is Clear In Stereo. If Not,
You Restart The Process
93. Microscopes
(Continued …)
To Use A Microscope, You Should Begin And End
With Low Power Zoom (10x), Using Higher
Powers To Focus And Identify Individual
Characteristics That Assist In Identifying An
Unknown Gem
94. Microscopes
(Continued …)
Although There Are Many Methods One May
Examine A Gem Through A Microscope, The
Method I Use Is To Mentally Break The Gem Into
Eight Pie Slices And Examine The Crown First
Starting From The Noon Position Going
Clockwise And Then Examine The Pavilion
Likewise
95. Microscopes
(Continued …)
If You Rotate A Gem On Its Horizontal Axis (Top
To Bottom), Then Inclusions That Were Present
On The Top Half Of The Gem When Viewing The
Crown Will Now Be Located In The Bottom Half
Of The Gem When Viewing The Pavilion And Vice
Versa For Inclusions In The Bottom Half
96. Microscopes
(Continued …)
If You Rotate A Gem On Its Vertical Axis (Left To
Right), Then Inclusions That Were Present On
The Left Half Of The Gem When Viewing The
Crown Will Now Be Located In The Right Half Of
The Gem When Viewing The Pavilion And Vice
Versa For Inclusions In The Right Half
97. Microscopes
(Continued …)
You Can Also Identify If A Gem Is Doubly
Refractive By Viewing The Facet Junctions
And/Or Inclusions Inside A Gem From At Least
Three Different Angles For The Presence Of
Doubling Due To The Splitting Of A Single Light
Wave Into Two Separate Waves In A Doubly
Refractive Gem
99. Microscopes
(Continued …)
It Is Also Possible To Compute The Refractive
Index Of A Gem Using A Microscope
(However, It Is Not As Accurate As A
Refractometer)
100. Microscopes
(Continued …)
First, You Measure The Actual Depth Of The
Gem Using A Micrometer (If Not Mounted) Or A
Leverage Gauge (If Mounted And Open
backed)
101. Microscopes
(Continued …)
You Then Place The Stone In A Microscope,
Making Sure That The Table Of The Stone Is
Parallel To The Microscope's Objective
You Adjust The Microscope To The Highest
Magnification It Can Achieve (The Higher The
Magnification, The Closer The R.I.) And Then
Focus The Microscope On The Gem's Top
102. Microscopes
(Continued …)
You Then Tape A Table Gauge To The
Stationary Side Of The Microscope Arm And
Make A Mark At Zero On The Movable Portion
You Then Focus The Microscope On The
Bottommost Point Of The Gem And Then Use
A Loop To Read And Record The Change In
Position Of The Mark On The Table Gauge
103. Microscopes
(Continued …)
To Compute The Approximate Refractive Index
For The Gem, You Divide The Actual Depth
Recorded With The Micrometer Or Leverage
Gauge By The Apparent Depth Recorded From
The Table Gauge On The Microscope's Arm
107. Spectroscopes
Spectroscopes Work On The Principle That
Certain Chemicals In Gems Absorb And Reflect
Certain Colors Of The Basic Visible Light
Spectrum Of Approximately 400nm (Blue) To
750nm (Red)
108. Spectroscopes
(Continued …)
The Chemical Components Of A Gem Show A
Basic Color Absorption Signature (With Minute
Variations Depending On Certain Trace Chemical
Elements Distinct To The Mining Environment)
Across A Color Prism Inside The Spectroscope
Which Can Be Compared With Known Signatures
110. Spectroscopes
(Continued …)
There Are Two Methods In Using A
Spectroscope Depending On The
Transparency Of The Gem In Question:
Transmission And Reflection
111. Spectroscopes
(Continued …)
If The Gem Is Transparent To Translucent,
Then You Should Place A White Light Behind
The Gem And View The Gem Through The
Spectroscope
112. Spectroscopes
(Continued …)
If The Gem Is Semi-Translucent To Opaque,
Then You Must Attempt To Read The Signature
By Reflecting A White Light Off The Surface Of
The Gem At A 45º Angle Into The Spectroscope
114. Ultraviolet Radiation Works On The Principle
That Certain Elements Within A Gem's
Chemical Structure Either Fluoresce And/Or
Phosphoresce In Reaction To UV Light
The Presence Of Fluorescence And/Or
Phosphorescence In Certain Gems Can Assist
In Determining An Identification
Ultraviolet Radiation
115. Ultraviolet Radiation
(Continued …)
You Check For Fluorescence In A Gem Under
The Presence Of Longwave And/Or Shortwave
UV Light In A Darkened Environment
You Check For Phosphorescence In A Gem By
Checking For The Existence Of Visible Color In
A Gem While In Darkness After The UV Light
Source Is Turned Off
116. Ultraviolet Radiation
(Continued …)
Gems Of The Same Species And/Or Variety
May Or May Not React To Ultraviolet Radiation,
So The Test Is Not Solid Proof Of Identity
117. Ultraviolet Radiation
(Continued …)
For Example, Myanmar Rubies Typically
Fluoresce Red. However, Thai Rubies
Typically Do Not Fluoresce Because Of The
High Iron Content Within Their Chemical
Makeup
119. The Chelsea Filter Works By Allowing Only
Certain Wavelengths Of Yellow-Green and Red
Colors To Be Transmitted Through The Filter
Gems That Transmit These Colors Will Appear
As Either Green Or Red, While Other Gems
Will Simply Appear Dark
Chelsea Filters
120. Chelsea Filters
(Continued …)
Chelsea Filters Are Used To Detect Dyes And
To Indicate The Coloring Agent In Some Gems
And To Separate Some Gem Materials From
Imitations
However, Certain Synthetic And Natural Gems
Appear The Same Through The Chelsea Filter,
So It Is Only A Supplemental Test
121. Chelsea Filters
(Continued …)
For Example, Natural Emerald Turns Red
When Viewed Through The Chelsea Filter;
However, Synthetic Emerald Also Turns Red
When Viewed Through The Chelsea Filter
In Addition, Demantoid Garnet And Some
Green Zircons Look Pinkish Or Reddish And
Can Be Easily Confused With Emerald
122. Chelsea Filters
(Continued …)
There Are Two Methods For Using The
Chelsea Filter: Transmission And Reflection
If A Gem Is Transparent To Translucent, Then
You Place A White Light Behind The Gem And
View The Gem Through The Chelsea Filter
123. Chelsea Filters
(Continued …)
If The Gem Is Semi-Translucent To Opaque,
Then You Direct A White Light At The Gem From
A 45º Angle And View The Reflection Through
The Chelsea Filter
125. Magnets Are A Useful Tool In Separating Some
Synthetic Diamonds From Natural Diamonds
Synthetic Diamonds Are Grown Using A
Metallic Flux To Speed Up The Process
Some Synthetic Diamonds Are Actually
Attracted To Magnets Due To Metallic Flux
Inclusions
Magnetism
126. Magnetism
(Continued …)
If You Find A Diamond That Is Attracted To A
Magnet, You Should Be Strongly Suspicious That
The Diamond Is Synthetic And Should Further
Test It Using UV Radiation And Looking For
“Hourglass” Or “Stop Sign” Growth Zoning And
Metallic Inclusions Under Magnification
129. Electrical Conductometer
Works On The Principle That In Some Diamonds
(Mostly Blue And Gray Type IIb Diamonds),
Electrons In The Gem Are Already Mobile And
Will Conduct Electrical Current Through The Gem
When It Is Applied
130. Electrical Conductometer
(Continued …)
The Instrument Is Useful For Determining If A
Diamond Is Naturally Blue And Gray Colored Or If
The Diamond Has Possibly Had Its Colored
Enhanced Or Added Due To Irradiation And/Or
Other Man-made Method
132. Scanning Electron Microscope
Is 100 Times Or More Powerful Than The
Highest Practical Optical Magnification (200x)
Of The Standard Stereo Gemological
Microscope
133. Scanning Electron Microscopes
(Continued …)
Works On The Principle Of Bombarding A Gem
With A Focused Beam Of X-rays And
Electrons, Causing A Secondary Emission Of
Electrons From The Gem's Surface Due To
Displacement
134. Scanning Electron Microscopes
(Continued …)
Useful When Coupled With A X-ray Detector To
Determine The Approximate Amounts Of The
Major Chemical Elements Present In A Gem
Each Chemical Element Emits A Characteristic
Amount Of Energy When Excited By The
Electron Beam, Which The X-ray Detector
Analyzes
136. Spectrophotometer (UV Through Visible Light)
Fluorescence Spectrometer (Fluorescence
Emitted By Gems Exposed To Ultraviolet And
Visible Light Radiation)
Spectrometry
137. Spectrometry
(Continued …)
Infrared Spectrometer (Infrared Radiation
Emitted By Gems Exposed To Visible Light
Through The Upper Edge Of The Radio Wave
Spectrum)
138. Spectrometry
(Continued …)
Similar In Range And Function To A
Spectroscope, The Spectrophotometer
Analyzes Electromagnetic Energy In Light
However, The Spectrophotometer Is Much
More Sensitive Than The Human Eye And Can
Detect Energy Present That The Human Eye
Can Not
139. Spectrometry
(Continued …)
The Spectrophotometer Works By Scanning The
Ultraviolet Through Visible Light Spectrum In Tiny
Nanometer Slices, Recording The Absorption
Pattern Reflected Back To The Instrument From
The Gem, And Displaying It In A Graph With
Peaks And Valleys According To The Light
Absorption
140. Spectrometry
(Continued …)
Although The Spectrophotometer Can Be Used
On Any Gem Or In Place Of Using A
Spectroscope, The Instrument Is Used Most Often
To Detect Treatment In Diamonds
141. Spectrometry
(Continued …)
The Fluorescence Spectrometer Is Similar To A
Spectrophotometer With The Exception That It
Measures The Fluorescence Emitted By Gems
Exposed To Ultraviolet And Visible Light
Radiation And Displays It On A Graph
142. Spectrometry
(Continued …)
Similar To A Fluorescence Spectrometer, The
Infrared Spectrometer Measures A Gem's
Absorption Of Infrared Radiation With The
Exception That Its Range Covers From Visible
Light Through The Upper End Of The Radio
Wave Region
143. Spectrometry
(Continued …)
Infrared Spectrometry Is Effective In Detecting
Substances Like Epoxy, Stain, Resin, And Plastic,
Which All Show A Characteristic Signature In The
Infrared Range, Without Having To Destroy Or
Mar A Gem To Retrieve A Sample
146. X-rays
Work On The Principle That Due To Their Short
Wavelengths And High Energy, X-rays Can
Penetrate Material That Ordinary And UV Light
Radiation Can Not
X-Rays Are The Only Method Of Positively
Separating The Different Types Of Pearls
147. X-rays
(Continued …)
Similar To Taking X-rays On Humans, Pearls
Subjected To X-rays From X-radiography Display
A Characteristic Signature Which A Trained
Laboratory Gemologist Can “Read” To Tell If They
Are Cultured Or Natural
148. X-rays
(Continued …)
Similar To UV Fluorescence, X-ray
Fluorescence Tests For The Emission Of
Energy, Visible Or Invisible, That Results From
Exposure To The Higher Energy Wavelength
Of X-rays
149. X-rays
(Continued …)
Visible Fluorescence To X-rays And Its
Strength Allows A Laboratory Gemologist
To Distinguish Between Saltwater And
Freshwater Pearls
150. X-rays
(Continued …)
Another X-ray Instrument In The Arsenal Of A
Lab Is Energy Dispersive X-ray Fluorescence
Energy Dispersive X-ray Fluorescence Works
On The Principle That X-ray Radiation Will
Stimulate The Emission Of Other Wavelengths
Which Are Themselves In The X-ray Region Of
The Radio Wave Spectrum
151. X-rays
(Continued …)
Energy Dispersive X-ray Fluorescence
Determines A Gem's Chemical Composition By
Analyzing The Characteristic Wavelength And
Amount Of Energy Each Chemical Element
Produces And Compares It To Known
Standards
152. X-rays
(Continued …)
X-ray Fluorescence Is Used Primarily To
Distinguish Between Natural And
Synthetic Emeralds And Rubies
154. Work On The Principle That Irradiated Gems,
Whether Natural Or Assisted By Man, Will Give
Off A Certain Amount Of Radiation Over A
Period Of Time Known As Half-life
Radiation Detectors Are Not Geiger Counters;
They Are More Sensitive Than Geiger Counters
Radiation Detectors
155. Radiation Detectors
(Continued …)
Most Irradiation Is Performed On Gems To
Achieve Color
It Is Important To Know When Gems Have
Been Bombarded With Irradiation Not Only For
Health Reasons, But Also Because Their Color
Can Be Altered With Heat From Routine
Repairs Done With A Bench Jeweler's Torch
157. The First Thing You Do To Begin The Gem
Identification Process Is To View The Unknown
Gem Without Magnification
You Should Note Color, Phenomena (Distinct
Visual Effects), Transparency, Clarity,
Fashioning, Luster, And Any Surface Breaks
Putting It All Together
158. Putting It All Together
(Continued …)
You Should Analyze Hue, Tone, And Saturation
Of The Color In The Unknown Gem
In Transparent Gems, Look For Any Color
Change In Either Or Between Daylight
Equivalent Fluorescent Lighting And
Incandescent Lighting
159. Putting It All Together
(Continued …)
In Transparent Gems, You Should Also Look
For Any Pleochroism, Color Zoning (Distinct
Bands Of Color) And/Or Dispersion (Different
Colors Of Light Exiting The Gem To Your Eyes)
In Translucent To Opaque Gems, You Should
Look For Color Banding, Mottling (Streaks Or
Patches Of Color), Or Any Distinct Markings
160. Putting It All Together
(Continued …)
You Should View Both The Top And Bottom Of
The Gem Looking For Any Distinct, Unnatural
Coloring Differences That Could Be A Strong
Indication Of An Assembled Stone
Look For Any Mold Marks Or “Orange Peel”
Effects Which Strongly Hint That The Gem Is
An Imitation
161. Putting It All Together
(Continued …)
Look At The Attention To Detail Left By The
Lapidary. Scratches, Abrasions, And Sloppy
Finishing All Point To Inexpensive Material
Pay Attention To Surface And/Or Fracture
Luster, Which Can Also Assist In Identifying An
Unknown Gem
162. Putting It All Together
(Continued …)
Under Magnification, Note Inclusions And
Check For The Doubling Of Facet Junctions
Within The Gem Using Darkfield Or Fiber Optic
Lighting
Using Overhead Lighting, Note Any Blemishes
Or Surface Conditions That Are Not Readily
Visible Without Magnification
163. Putting It All Together
(Continued …)
Take R.I. Reading(s) And, In Some Cases, Plot
The R.I. Readings To Determine Optic
Character
164. Putting It All Together
(Continued …)
If The Gem Is Transparent To Translucent, Use
The Polariscope To Determine If An Unknown
Gem Is Singly Refractive, Doubly Refractive, Or
An Aggregate. Remember, Do Not Attempt To
View Red Stones Over A R.I. Of 1.71
165. Putting It All Together
(Continued …)
If Possible, Determine The Optic Character
With The Polariscope If The Gem Is Doubly
Refractive
166. Putting It All Together
(Continued …)
If The Gem Is Transparent To Translucent,
Note Any Pleochroism Colors Through The
Dichroscope From At Least Three Different
Directions. Remember To Ignore Any Weak
Color Differences
167. Putting It All Together
(Continued …)
Armed With All The Previous Information You
Have Gathered, Look In Reliable References For
A Match. Starting With The R.I.(s), Narrow Your
Choices According To The Specifications For
Each Gem Species/Variety. If The Gem Species
Calls For An Additional Test Beyond The Primary
Tests, Perform It And Recompile The Results
168. Putting It All Together
(Continued …)
If The Information You Gathered From Primary
And Any Secondary Test(s) Does Not Match The
Information Provided In Reliable References, First
Reperform The Tests And Then Consider You
Have Either An Imitation Or You Have An
Anomaly. If It Is Not An Imitation, Refer The Gem
To GIA For Identification
169. Putting It All Together
(Continued …)
If You Are Not 100% Confident That You Have
Positively Identified An Unknown Stone (Such As
In The Case Of Synthetic Vs. Natural Gems),
Refer The Stone To GIA For Positive
Identification. No Wrong Identification Is Worth
The Consequences (Damaged Reputation,
Lawsuits, Etc.)
170. Putting It All Together
(Continued …)
Congratulations!
You Have Successfully Learned
How To Identify An Unknown Gem
171. Questions Or Comments
Scott B Britton, GIA Graduate Gemologist
Please Email Questions
Or Comments To
president@metdia.com
Or Visit Our Web Site At
https://www.metdia.com