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by gagan choudhary

                         
                          Chapter II
Geographical Locations of Gemstones
                          
                           (excerpt from pages 23 to 29)
 

Gem Belts and Gemstone Deposits in India and the World: The concept of gem belts and associated gemstone deposits has gradually evolved with a better understanding of the geological changes as a result of movement of land masses or the 'Continental Drift' theory. About 250 million years ago, there was a single land mass, which is referred to as 'Pangaea'. This land mass gradually separated, to form two large masses known as 'Gondwanaland' (comprising of South America, Africa, India, Antarctica and Australia) and 'Laurasia' (comprising of North America, Europe and Asia). With time each of the continents has separated to their current positions (Fig. 9). The geometric fit of one continent into another as seen between South America and Africa, gives credibility to this very important theory. This is also responsible for the explanation of the major gem belts in the world. This theory puts forward the association of the rock types, mineral associations and the geological processes which are observed in the various continents.

Figure 9: Breaking up of Pangaea to form the present day world

For the purpose of this book, it is sufficient to understand that there is a distinct correlation between the land masses, which is the reason for some gemstones having similar properties, being found in different locations. This creates a major problem for a Gemmologist, while trying to determine the origin of a gemstone.

There are a number of distinct Gem belts around the world, with some of the major ones being:


Southeast Asia which includes Sri Lanka, South East India, Myanmar (Burma), Thailand, Cambodia and Vietnam. All these localities are associated with metamorphic and basaltic deposits with granitic intrusions of pegmatites. Each of the individual countries is well known for their gem deposits and the numerous gemstones which are found there. Some of the important areas are Mogok district of Myanmar, Ratnapura in Sri Lanka, the states of Orissa, Andhra Pradesh, Karnataka and Tamil Nadu in India, Chantaburi-Trat province in Thailand and Luc Yen district in Vietnam.


East - South East Africa which includes Kenya, Tanzania, Zimbabwe and Zambia. The major belt in this region is the 'Mozambique Belt', which extends for at least 5000 kilometres - the complete eastern part of Africa, from North to South. Gemstones formed here are rich in chromium and vanadium. This is the reason why, a number of colour changing stones are found in this region such as garnet, sapphire and tourmaline. The most important deposits are located along the Umba valley in north east Tanzania, with others at Mgam Ridge and Mangari Swamp in South Kenya and the Morogoro region in east central Tanzania.


Eastern part of South America at Brazil, which is also considered as the 'Paradise of Gemstones', due to the fact that almost every gemstone is mined from here. The gemstones are found in pegmatites, hydrothermal veins and metamorphic rocks associated with mica schist.


Western part of North America where deposits are located at California, Montana, Wyoming, Arizona and Utah, and Canada.

There are several small or scattered deposits in Europe, Northwest and North Asia located in Pakistan, Afghanistan, Russia (Urals), Central India and the Himalayan region in North India. Other deposits are located in Australia especially the eastern part at New South Wales and Anakie Fields and also in China.


Geographical Locations of Gemstone Deposits: Gemstone deposits are an integral part of the Gem and Jewellery Industry and hence play a very important part. The availability of rough to a great extent defines the costing of the finished product. Needless to say a number of factors are associated, but the geographical location, access and availability are a major factor. Though not directly associated with the gemstone deposits, these factors are integral to the availability and the subsequent costing of the rough material, are the geographical location, the political situation in the region, the socio - economic factors distinct to each region, the climatic conditions and accessibility to the region and the individual government policies for mining and exploration.


Following are the geographical locations of deposits in India (Fig.10). The gem deposits in India are mainly pocket deposits and organised mining is being done currently in a few areas. The deposits have been explained State wise with an idea of the quality and status of the gem deposit.

Figure 10: Map of India showing major gemstone deposits

Jammu & Kashmir:
  • Corundum - Blue sapphire: Quality varies from a turbid white to a transparent deep blue. Availability is restricted due to climatic and political conditions. Located in the Kudi valley (Soomjam).
  • Other gemstones: Ruby, beryl, kyanite.


Rajasthan:

  • Garnet-pyrope-almandine: Quality varies from a dark blackish red to purple, all varieties, from a few carats to large boulders. Deposits are at Tonk, Bhilwara, Ajmer, Chittorgarh.
  • Beryl - aquamarine/yellow beryl/ emerald: Quality of emerald is low and is more in the nature of an exhausted deposit. Aquamarine colour ranges from colourless to greenish blue to moderate blue with a small percentage being of good transparent blue. Deposits are at Ajmer, Udaipur, Tonk etc.
  • Other gemstones: Rock crystal, amethyst, aventurine quartz, diopside, fluorite, garnet (almandine, hessonite), orthoclase feldspar, sodalite, vesuvianite etc.


Bihar / Jharkhand:

  • Plagioclase feldspar (labradorite, sunstone): Colour ranges from transparent colourless to opaque white and grey with prominent inky blue play of colour. Occurs in various sizes and larger boulders are commonly used for carving. Deposits are at Indarwa and Koderma.
  • Other gemstones are tourmaline, quartz cat's eye, diopside, beryl, epidote etc.

    Figure 11: Deep trench at Garnet mine, Tonk, Rajasthan                 

Madhya Pradesh / Chattisgarh: (These states have a number of new deposits and are fast becoming good suppliers of gemstones.)

  • Diamond: Colour range varies from colourless to brownish yellow to blackish with size variation from 0.01cts to 9cts, approximate gem quality is 50%. Deposits are located in kimberlite pipes at Panna (Fig.12), Tokapal and surrounding areas. Mines here are run by NMDC and Rio Tinto (mechanised & organised form of mining in India).
  • Corundum-ruby: Quality varies from pinkish red to a more purplish red with a large percent being cabochon material. Deposits are located in and around Raipur and Bastar districts (Chattisgarh).
  • Chrysoberyl - alexandrite: Colour change is about 65% with a yellow green in daylight to purple red in incandescent light. Sizes of rough are generally below 5cts. Availability is sporadic and in pegmatites. Deposits are at Deobhog in Raipur district.
  • Other gemstones: Pyrope-almandine garnet, yellow beryl, aquamarine, amethyst, yellow-green tourmaline etc.
 

Figure 12: Terrace mining at Panna mines, Madhya Pradesh

Orissa: (This state is a major source of gemstones)

  • Corundum- ruby & sapphire: Deposits are at Kalahandi and Bolangir districts. Colour varies from transparent to opaque, brownish red to purplish red, brown, yellow and blue sapphires - generally included qualities.
  • Chrysoberyl: Cat's eye ranges from a bluish green, brownish green, yellowish green, brownish yellow and honey gold colours. Alexandrite colour change is from bluish green to reddish brown, yellowish green to pinkish red, green to red. Deposits are located at Koraput, Bolangir, Kalahandi, Sambalpur and the area abutting Andhra Pradesh (Araku valley).
  • Beryl: Green, yellow and blue colours, transparent and cat's eye, sizes varying from 1ct. to over 500cts. Deposits are at Sambalpur, Phulbani, Bolangir.
  • Garnet: All quality ranges of pyrope-almandine (rhodolite), hessonite, pyrope-spessartine (colour changing), almandine-spessartine (malaya) and star almandine are met with. Most of the gem deposits are basically located in Koraput, Kalahandi, Bolangir, Phulbani and Sambalpur districts. 
  • Other gemstones include: Iolite, topaz, tourmaline, sillimanite, spinel, quartz varieties, diamond, feldspar (orthoclase, plagioclase), apatite, enstatite cat's eye, etc.                                                                                                                                                                               

                                                                                                                                                                                            Figure 13: Garnet mine at Kairamunda, Orissa

Andhra Pradesh:

  • Chrysoberyl - cat's eye & transparent variety: Quality of the cat's eye covers a wide range from a yellowish green to olive green to a bluish green with sizes varying from 1ct. to over 20cts. A lemon yellow cat's eye with a honey gold eye has been found as a small deposit. Deposits are located at Khamam, Araku valley and Vishakhapatnam.
  • Alexandrite and alexandrite cat's eye: Exhibit a bluish green to brownish red or pinkish red colour change (sometimes with a grey overtone), with sizes of rough ranging from 1ct. to about 10cts. Deposits are located at Khamam, Krishna river area, Araku valley and Vishakhapatnam.
  • Corundum -ruby: The quality varies from transparent to opaque reddish pink to red and more blackish red stars. Availability in large quantity, deposits are located at Khamam, Guntur, Anantpur, Warangal and Vishakhapatnam.
  • Other gemstones: Diamond, spinel, pyrope-almandine and hessonite garnet, amethyst, aventurine quartz, sillimanite, yellow green tourmaline, iolite etc.

Figure 14:  Chrysoberyl - alexandrite mines in Vishakhapatnam, Andhra Pradesh


Karnataka:

  • Corundum - ruby, sapphire: This state is well known for the continuous supply of star rubies over the years. The colour varies from a deep purple red to a pinkish red with the quality of the star also varying; sizes include one of the largest star rubies in the world weighing 6465cts. Deposits are located at Mysore, Ramannahalli, Dughahalli, Coorg, Mercara, Tumkur, Hassan, Chikmangalur. Transparent ruby from in and around Mysore have a pinkish red colour; blue sapphire which is highly included and opaque, more often known as blue corundum rock is found in large boulder sizes.
  • Other gemstones: Feldspar, kyanite, pyrope-almandine garnet, iolite, sillimanite, beryl, diopside, tourmaline, tiger's eye, hawk's eye, aventurine quartz etc.


Tamil Nadu:

  • Corundum- ruby: The colour varies from purple red to deep red with a fair percent of good quality. Deposits are located in the Kangyam-Karur belt, Salem, Manvadi, Kambam district, Coimbatore, Madurai and Tiruchirapalli districts. Some exceptional quality star ruby has been reported from Karur, sapphires in brown, grey, greenish yellow and black, some with asterism are also met with.
  • Feldspar: Orthoclase and plagioclase in all colours; moonstone namely white, grey, pink, green, black etc. from transparent to translucent, cat's eye and 4-ray star, sunstone feldspar - brownish to a pure orange colour and cat's eye variety, labradorite and spectrolite quality are moderate grades. Deposits are located in Karur, Salem, Coimbatore, Madurai etc.
  • Chrysoberyl: Alexandrite and cat's eye deposits are located in the Kangyam-Karur region and Madurai district, colour change in alexandrite is from a deep green to blood red colour in the finer specimens and a yellowish green to brownish red in the moderate qualities. Availability is very sporadic. The cat's eye colour range is a yellowish green to green.
  • Other gemstones: Black star diopside, beryl (aquamarine, emerald), quartz (rock crystal, rose, aventurine, sagenitic quartz), iolite (bloodshot iolite, cat's eye), sphene, zircon, enstatite cat's eye, etc  


Kerala:

  • Chrysoberyl: Cat's eye and alexandrite of good quality. Greenish yellow, grey to brownish yellow cat's eye. The colour change is from a yellowish/brownish green to a brownish red. Deposits are located at the border regions abutting Tamil Nadu and are sporadic.
  • Other gemstones: Corundum (ruby & sapphire), feldspar (orthoclase and plagioclase), topaz, quartz (star, cat's eye, rose, sagenitic quartz), beryl (aquamarine) etc.