GEMSTONES

GEMSTONES: 

• The minerals with special beauty and physical properties which attract attention to possess them for personal adornment and decorative purposes are gemstones.

      

• Beauty , rarity and durability are the main virtues of a gemstone. They are hard enough to resist the mechanical and chemical actions of every day's life . 

• The cut stones are known as Gems while the Uncut - ones are gemstones.

• The typical characteristics of gems are sheen , chatoyancy , opalescence , iridescence and dichroism.  

      

• The gemstones may be grouped into -

 

• Precious stones namely Diamond , emerald, Sapphire , ruby , precious opal and pearl.

      

• Semi- precious stones which include Aquamarine (beryl) ,  Moonstone (felspar), Amethyst (quartz), peridote  (olivine), and a good no. of minerals.

      

• The standard international weight of gems are in metric carat which is 1/5 of gramme (200 mg) . For pearls , the unit of a weight is a pearl grain (1/4 carat or mg). 

     

• India is  the largest exporter of cut and polished gems  (70% of world's sale). Depend on imported raw material. 

     

• The diamond and gem development Corporation of India have set up diamond and gem parks for cutting and polishing of gems at Sachin near Surat (Gujarat) , Jaipur (Rajasthan) , Haldwani (U.P), Tiruchi (Tamil Nadu) and Govindpur near Jehanabad (Bihar).

                                              

CLASSIFICATION AND IDENTIFICATION:

• The gems may be classified based on their colour , transparency, chemical composition , Specific gravity and host rock.

• The properties like Hardness , Refractive index , Dichroism and ultraviolet tests besides Specific gravity are employed in identification of gems.

•       PROPERTIES FOR CLASSIFICATION AND IDENTIFICATION

USES:

• The gemstones find wide application in jewellery . The semi - precious stones apart from jewellery are used as ornamental stones for wall decoration , floors of building , vases , statuettes.

      

• Some of which are neither useful for gems nor for ornamental purposes are used as abrasives where hardness is taken into accounts.

       

• Diamond which is a primarily a gemstone , is also used as industrial stone.

• Rubies and Sapphire are used as watch jewels.

• Quartz and Tourmaline are used in optical instruments.

• The balance bearings and the nortars  and pestles are made up of agate.

• The synthetic stones may be manufactured by fusing pure alumina.  

MODE OF OCCURENCE AND ORIGIN:

• Gemstones are found dominantly in igneous rocks , Granitic Pegmatites and as Detrital grains in alluvial . They occur as veins (quartz) , in cavities (opal , agate , zeolites etc.) in volcanic pipes (diamond) and in variety of ways.

• During crystallisation of magma the magnesium olivine (peridot) and the ultra basics are first to separate out . Some of these ultra basics occupy volcanic vents as pipes or dykes giving rise to diamondiferous kimberlites with crystallisation of diamond, pyrope and olivine.  

      

• The remaining magma becomes enriched in silica with lithium, beryllium boron and volatile constituents like fluorine , chlorine and hydroxyl and result into pegmatites with large crystals of quartz , felspar and mica with gems like tourmaline, beryl , topaz

• Ruby and Spinel  are formed due to contact metamorphism of impure limestone.

      

• Nephrite, Staurolite, kyanite and Sillimanite result  under regional metamorphism.

• Lepis- lazuli , Serpentine and thomsonite are formed by the action of volatile constituents on intruded rocks .

• Hydrothermal solutions yield opal and agate , while supergene processes give rise to turquois.

• The gemstones get concentrated in the alluvial and massive placers by weathering and transportation.

• These are quite important source of gemstones from economic point of view. 

DISTRIBUTION:

• Distribution Table

RESOURCES AND PRODUCTION:

• Mica- Pegmatites and metamorphic suite of rocks with kyanite, sillimanite, corundum, garnet etc like schists , Khondalites and equivalents formations are the main sources for gemstones.

     

• Some of the semi -precious stones occur in trap rocks.

      

• The alluvial placers formed by marginal streams bordering the granite masses with pegmatites and metamorphosed sediments are considered most viable sites for the gemstones.

• Resources and Productions

A brief description of some gems is given below:

1.  Sapphire and Ruby : They are gem varieties of corundum and are respectively blue and red in colour. The other gem types of corundum  and oriented amethyst , oriented emerald and oriental topaz which are purple , green and yellow respectively.  

2. Emerald: Emerald is transparent pale- green to sea- green variety of beryl.

3. Semi- precious silica group of minerals : Agate , amethyst , Fuchite and Jasper are the semi- precious minerals, belonging to silica group . They are described below.

AGATE:

• Agate is a variety  of  Chalcedony , showing regular or irregular bands with pleasant colours. Agate pieces after cutting and polishing are sold as semi - precious stones . 

• Big pieces are used in making mortars and pestles for laboratory use. Agate pieces cut into requisite shapes are sold as fulcra of scientific balances and making edges , planes and bearings of precision instruments. 

• Chalcedony Pebbles are used as balls in the ball mills for crushing and grinding felspar , calcite and barytes.  

      

• It occurs mainly as fillings in the voids in the deccan trap rocks.

           

         

             

AMETHYST:

• It is used as semi - precious stone and in making curios . It is mined in penimalai R.F. area in Madurai district Tamil Nadu. 

          

JASPER:

• Jasper, a Chalcedonic variety of silica is usually a deep red and brownish red colour and is used for making decorative stone. Jasper mining is being carried out mainly in jodhpur district of Rajasthan. 

                     

Fuchsite Quartzite:

• Fuchsite (green quartzite) is utilised for making curios, paper weights , hand cuffs , studs and similar other artifacts. It is mined in Chitra Durga district of Karnataka and Anantapur district of Andhra Pradesh.