gemology

1. Silica Group of GemstonesSilica Group of Gemstones
 Of all gemstone varieties, silica group are mostOf all gemstone varieties, silica group are most
widely distributed and abundant.widely distributed and abundant.
 Known as poor Man’s gem.Known as poor Man’s gem.
 Silica occurs in three forms:Silica occurs in three forms:
 Phanerocrystalline,Phanerocrystalline,
cryptocrystalline/Microcrystalline andcryptocrystalline/Microcrystalline and
Amorphous.Amorphous.
 Silica or SiOSilica or SiO22 is the best compound to illustrateis the best compound to illustrate
phenomenon of polymorphism.phenomenon of polymorphism.

2.  Depending on P-T conditions crystallizes intoDepending on P-T conditions crystallizes into
different polymorphs.different polymorphs.
 Common for of SiOCommon for of SiO22------ α-quartz or low------ α-quartz or low
temperature quartz simply called as “quartz”.temperature quartz simply called as “quartz”.
 Gem verities quartz belong to this polymorph.Gem verities quartz belong to this polymorph.
 In gem trade, clear colourless quartz is calledIn gem trade, clear colourless quartz is called
“rock crystal”.“rock crystal”.
 Phenerocrystalline form of silicaPhenerocrystalline form of silica
 Dimensions of large crystals (about 1mm orDimensions of large crystals (about 1mm or
more) to be seen by unaided eye.more) to be seen by unaided eye.

3.  Cryptocrystalline silica consists of minute fibresCryptocrystalline silica consists of minute fibres
or granular mass of quartz. Individual quartzor granular mass of quartz. Individual quartz
crystals cannot be seen without aid of acrystals cannot be seen without aid of a
microscope (thin section)microscope (thin section)
 Amorphous silica forms are natural glassesAmorphous silica forms are natural glasses
called “fulgarite”.called “fulgarite”.
 Crystal system Hexagonal.Crystal system Hexagonal.
 In rocks---- massive form.In rocks---- massive form.
 Grown in cavities --- develop into prismaticGrown in cavities --- develop into prismatic
crystals of hexagonal forms.crystals of hexagonal forms.

4. fulgaritefulgarite

5.  Although there are indistinct cleavageAlthough there are indistinct cleavage
planes in quartz, for all practical purposesplanes in quartz, for all practical purposes
quartz has no cleavages.quartz has no cleavages.
 Fracture ----conchoidal to subconchoidalFracture ----conchoidal to subconchoidal
 Lustre –vitreousLustre –vitreous
 Hardness ---7Hardness ---7
 Specific Gravity---2.65Specific Gravity---2.65
 Dissolves only in hydrochloric acid.Dissolves only in hydrochloric acid.

6. Varieties of QuartzVarieties of Quartz
 Uncoloured quartz occurs almost as aUncoloured quartz occurs almost as a
pure compound SiOpure compound SiO22..
 Impurities in this minerals may be in ppm.Impurities in this minerals may be in ppm.
 In colour quartz varieties -----impuritiesIn colour quartz varieties -----impurities
amount for only a fraction of a percent.amount for only a fraction of a percent.
 Even in the darkest coloured quartzEven in the darkest coloured quartz
impurities do not exceed 1%.impurities do not exceed 1%.
 Fluid inclusion are also common in quartz.Fluid inclusion are also common in quartz.

7.  Al, Ti, Fe, are common element asAl, Ti, Fe, are common element as
chromophores.chromophores.
 Mn, Li, Na, K, and Ca may also beMn, Li, Na, K, and Ca may also be
present.present.
 The most important quartz gemstones.The most important quartz gemstones.
Rose quartz, Amethyst, Agate, and onyx.Rose quartz, Amethyst, Agate, and onyx.
 Rose quartz, Amethyst---Rose quartz, Amethyst---
Phanerocrystalline.Phanerocrystalline.
 Agate and Onyx----- cryptocrystalline.Agate and Onyx----- cryptocrystalline.

8. Rose quartzRose quartz
 Pink to rose red in colour. Deeply colouredPink to rose red in colour. Deeply coloured
stones are also common. The colour maystones are also common. The colour may
often fade on exposure to sunlight. It isoften fade on exposure to sunlight. It is
generally translucent to sub translucent.generally translucent to sub translucent.
Transparent are rare. Colour may be dueTransparent are rare. Colour may be due
to TiOto TiO22 in traces.in traces.

9. AmethystAmethyst
 Most attractiveMost attractive phanerocrystallinephanerocrystalline quartz variety.quartz variety.
 Shades of colour---bluish violet to purple.Shades of colour---bluish violet to purple.
 Deeply coloured opaque stones appear almost black.Deeply coloured opaque stones appear almost black.
 Causes of colour – due to FeCauses of colour – due to Fe22OO33 impurity or MnOimpurity or MnO22..
 Heating amethyst ---- 400 to 450Heating amethyst ---- 400 to 45000
C converts to reddishC converts to reddish
brown called burnt amethyst. Resemble garnetbrown called burnt amethyst. Resemble garnet
(Gomedhikam). On further heating colour may be(Gomedhikam). On further heating colour may be
bleached to milky opalescence.bleached to milky opalescence.
 Aventurine quartzAventurine quartz: a quartz containing spangles of: a quartz containing spangles of
mica, haematite etc.mica, haematite etc.

10. Aventurine quartzAventurine quartz

11. AgateAgate
 Agate is a fibrous variety ofAgate is a fibrous variety of
cryptocrystalline silica composed ofcryptocrystalline silica composed of
different coloured concentric bands withdifferent coloured concentric bands with
sharper diffused boundaries.sharper diffused boundaries.
 If agate consists of green coloured chloriteIf agate consists of green coloured chlorite
in patches or in vermicular form, then it isin patches or in vermicular form, then it is
known as “mass Agate” or “Mocha Agate”.known as “mass Agate” or “Mocha Agate”.

12.  A banded agate of contrasting colour setA banded agate of contrasting colour set
in clearly into an “ eye” design, then it isin clearly into an “ eye” design, then it is
known as “Eye Agate”.known as “Eye Agate”.
 Agate with straight bands of contrastingAgate with straight bands of contrasting
colour is known as “onyx”.colour is known as “onyx”.

13. JasperJasper
 Jasper is an impure opaque form ofis an impure opaque form of
cryptocrystalline silica, usually of red,cryptocrystalline silica, usually of red,
brown and yellow colours, rarely green. Itbrown and yellow colours, rarely green. It
is opaque, even in thinnest edges. Someis opaque, even in thinnest edges. Some
varieties such as beautifully banded withvarieties such as beautifully banded with
different shades of brown are called asdifferent shades of brown are called as
Egyptian or Ribbon Jasper.Egyptian or Ribbon Jasper.

14.  Quartz is the most common mineral in acid andQuartz is the most common mineral in acid and
intermediate igneous rocks , sedimentary rocksintermediate igneous rocks , sedimentary rocks
like sandstones and conglomerates. Also inlike sandstones and conglomerates. Also in
schist and gneiss. Owing to its hardness, alsoschist and gneiss. Owing to its hardness, also
occurs detrital minerals in river bed and seaoccurs detrital minerals in river bed and sea
shores.shores.
 Concentration of silica in later stage of magmaticConcentration of silica in later stage of magmatic
crystallisation, hence occurs in pegmatites,crystallisation, hence occurs in pegmatites,
hydrothermal deposits in vaps, fissures andhydrothermal deposits in vaps, fissures and
cavities etc Amygdoles and geodes in floodcavities etc Amygdoles and geodes in flood
basalts.basalts.

15.  Quartz and its different forms occur inQuartz and its different forms occur in
several localities in Deccan basalt regionseveral localities in Deccan basalt region
of Maharashtra, Gujarat and Andhraof Maharashtra, Gujarat and Andhra
Pradesh.Pradesh.
 Amethyst and agate occur in severalAmethyst and agate occur in several
localities with mica pegmatites in Bihar,localities with mica pegmatites in Bihar,
Andhra Pradesh and Rajasthan.Andhra Pradesh and Rajasthan.