Silica Group of GemstonesSilica Group of Gemstones Of all gemstone varieties, silica group are most Of 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 and cryptocrystalline/Microcrystalline and Amorphous.Amorphous.

Silica or SiOSilica or SiO22 is the best compound to illustrate is the best compound to illustrate phenomenon of polymorphism.phenomenon of polymorphism.

Depending on P-T conditions crystallizes into Depending 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 called In gem trade, clear colourless quartz is called

“rock crystal”.“rock crystal”. Phenerocrystalline form of silicaPhenerocrystalline form of silica Dimensions of large crystals (about 1mm or Dimensions of large crystals (about 1mm or

more) to be seen by unaided eye. more) to be seen by unaided eye.

Cryptocrystalline silica consists of minute fibres Cryptocrystalline silica consists of minute fibres or granular mass of quartz. Individual quartz or granular mass of quartz. Individual quartz crystals cannot be seen without aid of a crystals cannot be seen without aid of a microscope (thin section)microscope (thin section)

Amorphous silica forms are natural glasses Amorphous 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 prismatic Grown in cavities --- develop into prismatic

crystals of hexagonal forms.crystals of hexagonal forms.

fulgaritefulgarite

Although there are indistinct cleavage Although there are indistinct cleavage planes in quartz, for all practical purposes planes in quartz, for all practical purposes quartz has no cleavages. quartz has no cleavages.

Fracture ----conchoidal to subconchoidalFracture ----conchoidal to subconchoidal Lustre –vitreous Lustre –vitreous Hardness ---7 Hardness ---7 Specific Gravity---2.65Specific Gravity---2.65 Dissolves only in hydrochloric acid.Dissolves only in hydrochloric acid.

Varieties of QuartzVarieties of Quartz Uncoloured quartz occurs almost as a Uncoloured 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 -----impurities In 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 quartz Even 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.

Al, Ti, Fe, are common element as Al, Ti, Fe, are common element as chromophores.chromophores.

Mn, Li, Na, K, and Ca may also be Mn, 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.

Rose quartzRose quartz Pink to rose red in colour. Deeply coloured Pink to rose red in colour. Deeply coloured

stones are also common. The colour may stones are also common. The colour may often fade on exposure to sunlight. It is often fade on exposure to sunlight. It is generally translucent to sub translucent. generally translucent to sub translucent. Transparent are rare. Colour may be due Transparent are rare. Colour may be due to TiOto TiO22 in traces. in traces.

AmethystAmethyst Most attractive Most 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 MnO impurity or MnO22. . Heating amethyst ---- 400 to 450Heating amethyst ---- 400 to 45000C converts to reddish C converts to reddish

brown called burnt amethyst. Resemble garnet brown 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.

Aventurine quartzAventurine quartz

AgateAgate Agate is a fibrous variety of Agate is a fibrous variety of

cryptocrystalline silica composed of cryptocrystalline silica composed of different coloured concentric bands with different coloured concentric bands with sharper diffused boundaries.sharper diffused boundaries.

If agate consists of green coloured chlorite If agate consists of green coloured chlorite in patches or in vermicular form, then it is in patches or in vermicular form, then it is known as “mass Agate” or “Mocha Agate”.known as “mass Agate” or “Mocha Agate”.

A banded agate of contrasting colour set A banded agate of contrasting colour set in clearly into an “ eye” design, then it is in clearly into an “ eye” design, then it is known as “Eye Agate”.known as “Eye Agate”.

Agate with straight bands of contrasting Agate with straight bands of contrasting colour is known as “onyx”.colour is known as “onyx”.

JasperJasper

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

Quartz is the most common mineral in acid and Quartz is the most common mineral in acid and intermediate igneous rocks , sedimentary rocks intermediate igneous rocks , sedimentary rocks like sandstones and conglomerates. Also in like sandstones and conglomerates. Also in schist and gneiss. Owing to its hardness, also schist and gneiss. Owing to its hardness, also occurs detrital minerals in river bed and sea occurs detrital minerals in river bed and sea shores.shores.

Concentration of silica in later stage of magmatic Concentration of silica in later stage of magmatic crystallisation, hence occurs in pegmatites, crystallisation, hence occurs in pegmatites, hydrothermal deposits in vaps, fissures and hydrothermal deposits in vaps, fissures and cavities etc Amygdoles and geodes in flood cavities etc Amygdoles and geodes in flood basalts.basalts.

Quartz and its different forms occur in Quartz and its different forms occur in several localities in Deccan basalt region several localities in Deccan basalt region of Maharashtra, Gujarat and Andhra of Maharashtra, Gujarat and Andhra Pradesh.Pradesh.

Amethyst and agate occur in several Amethyst 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.