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:
Bihar / Jharkhand:
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.)
Figure 12: Terrace mining at Panna mines, Madhya Pradesh
Orissa: (This state is a major source of gemstones)
Figure 13: Garnet mine at Kairamunda, Orissa