Gemstone Exploration & Mining

Geologist
&
Gemological Sciences

By
S.G SURENDER
SOMIKA S.P.R.L
sgs_sena@rediffmail.com

Resources are not; they become.
--- E.S Zimmerman

“A proper appreciation of mineral
resources may be the greatest service
we as geologist can perform for our
society and country”.
---E.L Chile.

Introduction
 Nearly 4000+ varieties of minerals are identified in
nature; only a few are referred as gem minerals
due to their exceptional qualities and properties.

 Gems have been a part of human history for more
than 5000 years. Throughout the ages gemstones
have been seen as representations of wealth, power
and symbols of supremacy.

 Gem minerals are usually hard and relatively free
from cleavage and occur as transparent crystals, with
few exception such as opal, fluorite etc.,

 Thus gemstones are artificially polished fragments
(faceted or with smooth curved finish) that are used
for decorative purposes and personal adornment.

 The study of gemstones is an accepted part of
Mineralogy .

 A gem is a naturally occurring material
desirable for its beauty, valuable in its
rarity, and sufficiently durable to give
lasting pleasure.

 Gemology- is the subject matter the
scientific study of gemstones (from its
genesis to valuation).

 Lapidary – is the science of cutting and
polishing a gem mineral.

 Over the centuries numerous magical and
medicinal properties were attributed to precious
stones.

 In Ayurveda (Indian System of Medicine)
 Audbhida-Extract form Plants,
 Jangama-Extract from Animals
 Parthiva-Extract from Metals and Minerals

Different types of BHASMA such as Hirka Bhasma
(Diamond), Manikya Bhasma (Ruby), Swarna
Bhasma (Gold), Abhraka Bhasma (Mica) etc. were
prepared from metals and minerals to cure variety
of diseases.

CLASSIFICATION OF GEMSTONES

• Gemstones are classified based on certain
assertive aspects /parameters. viz.,

1. TYPES

Organic Imitation

Inorganic Synthetic

2. Based on Availability

Availability

Precious Semi-Precious

The precious and semi precious classification of
gemstones is largely based on the Market (Demand
and Supply); it is an arbitrary classification based on
local availability. sgs_sena@rediffmail.com

3. OTHER TYPE CLASSIFICATIONS

Chemical Optical
Composition Characters

Durability
Crystal
Structure


Physical and Optical properties are the Key elements
to distinguish various types of gemstones
Physical Properties Optical Properties
• Color * • Refractive Index
• Streak
• Habit
• Dispersion
• Cleavage, Parting Plane, • Pleochroism
Fractures. • Interference Figures
• Lustre (Reflectivity) • Optic Axis
• Diaphaneity (Transmisivity) • Optic Sign
• Hardness*
• Specific Gravity
• Tenacity & Toughness
• Chatoyancy
• Asterism

COLOUR CAUSING TRANSITION METAL
ELEMENTS IN IDOCHROMATIC GEMS

Chromium Green Uvarovite
Iron Green Peridot
Red Almandine
blue lazurite
Manganese Pink Rhodocrosite,
orange Rodonite
Spessartite
Copper Blue Azurite, Turquoise,
Green Chrysocolk
Red Malachite, Dioptase
Cuprite

COLOUR CAUSING TRANSITION METAL ELEMENTS
IN ALLOCHROMATIC GEMS
Chromium Green Emerald, Gromularite, Cr-tourmaline,
Cr-Chalcedony, Alexandrite
Red Ruby, Spinal, Topaz, Alexandrite

Chromium Red Pyrope
+ Iron Green Jadeite, Diopside, Synthetic OPX
Cobalt Blue Syn. Spinel, Syn. Quartz & Glass.
Iron Green Aquamarine, Tourmaline, Green
Sapphire,
Yellow Chrysoberyl, Epidote. Yellow sapphire,
Citrine,
Blue Chrysoberyl,
Violet – Orthoclase, Soundmen.
blue Aquamarine, Spinal. Tanzanite, Iolite.
Red Eudialyte

Iron + Blue Sapphire
Titanium
Manganese Pink Rhodonite,
Yellow - Rhodocrosite,
Green Piedmmtite, Syn. OPX.
Violet Andalusite
Tremolite
Nickel Green Syn. Olivine.
Vanadium Green V- Emerald
Blue – Green V- Grossularite
to V- Sapphire
Purple – red Syn. CPX
Blue

The main parameters of GEMSTONE

• BEAUTY

• RARITY

• DURABILITY

 Gemstones are measured in terms metric carat,
which is equivalent to 200mg (1 carat is equivalent to
0.2 grams). A CARAT is again sub divided into
hundred POINTS (2mg).
 Gem Testing & Gemologist Certificate:
Gemstones are identified based on their properties (P
& O) along with few instruments (Chelsea filter,
Refractrometer, Gemological Microscopes, Hardness
Pencils etc.). The efficiency of a gemologist lies in
identifying a stone without subjecting it to any kind
of destructive tests.
 A certificate from a certified GEMOLOGIST would
certainly yield to its market, through proper
identification and valuation.

Valuation of Gemstones
valuation of gemstones is done through the
4 C’s precept.

 Color

 Clarity

 Cut &

 Carat Weight

EXPLORATION
 Exploration: Refers to geological, geochemical and
geo-physical field work and complimentary laboratory
studies directed to the discovery of workable mineral
concentration.

 Gemstone occurrence is a very rare and a transient
geologic feature; once exhausted cannot be
replenished with in a short time.

 Diverse minerals are formed at different geological
environments in different rock types, viz Igneous,
Sedimentary and Metamorphic rocks, also few
organically. sgs_sena@rediffmail.com

For Example:
 Diamonds in Kimberlites (Ultra-mafic HTHP)

 Beryl in Pegmatites

 Quartz in Felsic rocks

 Peridot in Ultra mafic rocks

 Garnets in Metamorphic Rocks

T h e p r in c ip le r o le o f a n
E x p lo r a t io n G e o lo g is t

Hos t
Roc k

E s s e n t ia l
M in e r a l

G e m Q u a lit y E m e r a ld A lm a n d in e G a r n e t
C r y s t a l in Q u a r t s a n d sgs_sena@rediffmail.com e m b e d d e d in

BERYL
 By the name Beryl is a Silicate of Beryllium and
Aluminum; is the principle store of beryllium in the
earth's crust. In its rare gem form it is notable for a
variety of vivid allochromatic color with vivid names.
 Chemical Composition: Be3Al2Si6O18
• Crystal System: Hexagonal
• Habit: Usually well formed hexagonal prisms, either
flattened or elongate, with pinacoidal terminations.
• Hardness: 7.5-8
• Specific Gravity: 2.67-2.78
• Refractive Index: 1.57-1.59

Distinguishing Properties
• Low R.I
• Low S.G*
• Dichroism
• Low Birefringence
Distribution:
Common rocks that hosts Beryl are Granites and
Pegmatites.
Vugs in these rocks may consists of fine crystals of
Beryl, Tourmaline, Topaz, Spodumene etc.
Mineralization occurs, where the Pegmatites intruding
the basis and ultra-basic rocks yield important gem
deposits*.

Varieties of Beryl
Sl No. Variety Color Cause
1 Goshenite Uncolored No Specific Cause
2 Heliodor Yellow UVCT (O2-- Fe3+)
3 Aquamarine Blue IVCT (Fe2+  Fe3+)
4 Green Beryl Green UVCT+IVCT
5 Morganite Pink Mn2+
6 Bixbite Red Mn2+
7 Maxixe Blue Color Centre (No3)
8 Maxixe -typed Blue Color Centre (Co3)
9 Emerald Green Cr3+/ V3+
(Velvety)

Emerald

Velvety green variety of Beryl
if known as Emerald.
CC: Be3(Al,Cr)2 Si6O18
The typical velvety green hue
of emerald is distinctive.
The green shade of emerald
caused on the account of
chromium/vanadium impurity.
RI- 1.578-1.584;
 SG- 2.72


Aquamarine

• Palish blue, light blue green
variety of beryl is known as
aquamarine.

• Sky blue color stone is the most
valuable aquamarine.

• They exhibits a greenish blue
tinge due the phenomenon of
pleochroism.


• Golden colored variety of beryl
is known as Golden beryl.

• The yellow color is due to the
presence of Fe3+ in a
concentration of over 1%.

• Yellow green variety of beryl is
known as Heliodor.

• Yellow green olive color is the
typical character of Heliodor.

• Bixbite is an extremely rare
variety of beryl.

• Bixbite is strong ruby red or
violet red in color.

• Very rarely found, have an
different genesis to that of
found in pegmatite. In
extrusive magmatic rocks.

• The red color is due to the
presence of Mn2+ is as an
impurity in their crystal
structure.

Case Study: Ndola Rural Emerald
Fields, Zambia
• Nolda –Rural Emerald field of the Zambian Republic
represents a good source of easily mined emeralds.
• To this day, many areas are a preserve of artisinal
miners. Lack of capital has lead to inefficient
exploration and exploitation.

GEOLOGICAL SETUP:
• Zambia lies in between Zaire and the Zambian cratons
characterized geologically as being ancient mobile
belt; comprising of Igneous, Sedimentary and
metamorphic formations of the Precambrian age.

• Mineralization in the Ndola Rural Emerald Fields
occurs where the quartz-tourmaline-pegmatite
veins intrude the talc-chlorite-tremolite-magnetite
schist and alter the contact zone as biotite-phaloghite
rock, this alteration zone forms the chief ore body.

Three distinct varieties of pegmatite are known in the
study area:
• 1st Generation: initial feeder phase rich in quartz
and feldspar and contain minor fine grained
tourmaline and not related to mineralization.
• 2nd Generation: veins composed largely of
tourmaline commonly associated with emerald
mineralization.

• 3rd Generation: composed of coarse quartz
tourmaline (5-10 mm) always found in association
with mineralization of emerald.
• The distribution of emerald is very erratic; major
emerald mineralization, rich in volume & grade are
associated with flat lying undulating veins and
number of vein intersects.

• Granitic emplacement characterized by intensive
pulses of pegmatite activity rich in minerals B, Be, F
etc intruding through the well developed shear
system appears to be the cause.
• Crystallization of beryl appears to have taken place
during metasomatic reaction between the slow
cooling pegmatite (3rd Generation) and the
surrounding schist.

CONCLUSION


The role of a GEOLOGIST in the field of
Gemological Science:
• Prospecting of a gem deposit.
• Establishing the properties of gem minerals.
• Identification of stone in cut and rough form.
• Enhancement of stone (synthesis).
• Minerals grow in a definite physico-chemical
environment, thus a geologist is better able
to understand and reason out the paragenesis
of a mineral or gemstone. It is easy to
distinguish a natural stone from its synthetic
counterpart. sgs_sena@rediffmail.com

Among the natural green gemstones
hydrogrossular, dematoid, chrome
diopside, chrome-tourmaline, peridot,
may be confused for emerald.


“Gems are the flowers of the
mineral kingdom, exquisite products
of Nature which have given a great
deal of satisfaction, to those who
behold them. They have fascinated
mankind for thousands of years”.

-Dr. B.P Radhakrishna

GEOLOGY ILLUMINATES THE PAST , SUSTAINS
THE PRESENT AND PROMOTES THE FUTURE.

Re f e r e n c e s :
• Bateman A.M., Tensen M.L., (1982) Economic Mineral
Deposits, Third Edition, John Willy & Sons pp/

• Curizio Cipriani., Alessandro Borelli., (1992) Gems and
Precious Stones, Simon & Schuster Inc, pp. 10-49,
138-149.

• Emerald Deposit of Nolda, Zambia (1997),
www. members.tripod.com/emerald/html.

• O. Von Knorring., (1970) Mineralogical and
Geochemical Aspects of Pegmatites from Orogenic
Belts of Equatorial and Southern Africa, African
Magmatism and Tectonics, Oliver Boyd Publishers

• Guilbert J.M., Park C.F., (1988) Ore Geology, W.H
Freeman Company, New York, pp. 487-511.

• Hall Cally., (1995) Gemstones, Dorling Kindersley Publ.

• Joel. E Arem., (1987) Color Encyclopedia of Gemstones,
Van Nostrand Reinhold Company, pp 1-20.

• Karanth R.V., (2000) Gems and Gemstone Industry in
India, Geological Society of India, pp. 1-56, 233-242.

• Klein Cornelis., Hurlbut S Cornelis Jr., (1993) Manual of
Mineralogy, John Wiley & Sons, pp. 593-605.


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