silica group of gemstones

silica group of gemstones

• 1.
  Silica Group ofGemstonesSilica 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.
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   Depending onP-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.
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   Cryptocrystalline silicaconsists 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.
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  fulgaritefulgarite
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   Although thereare 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.
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  Varieties of QuartzVarietiesof 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.
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   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.
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  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.
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  AmethystAmethyst  Most attractiveMostattractive 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.
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  Aventurine quartzAventurine quartz
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  AgateAgate  Agate isa 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”.
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   A bandedagate 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”.
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  JasperJasper  Jasper isan 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.
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   Quartz isthe 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.
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   Quartz andits 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.