Gem Stones:
There are over 3000 minerals in the world. Out of those minerals, 50 are used as gemstones. The others are too soft and easily scratched. So that they are not suitable for wearing. Gemstones are beautiful, rare, durable and hard enough to use.
Gemstones are found in gem gravels or in rocks. Rocks are made up of minerals and can be divided in to three types.
- Igneous
- Sedimentary
- Metamorphic
The formation of these three types is a continuous process. Gem quality minerals in these rocks are easily lie on earth’s surface, or lie buried deep on it. Other rocks; separated from their mother rock (is called host rock) by erosion, are carried to lakes or to the sea, by rivers.
Igneous Rocks:
These forms as solidified from molten rock; erodes and is deposited as sediments, which comes from volcanoes as lava. Some igneous rocks are coming out from lava as volcanic bombs or ash. These are solidified beneath the surface. When these rocks cool and solidifies very slowly the larger crystals are formed and therefore the gemstones are formed within that rock. These kind of igneous rocks are known as pegmatite.
Sedimentary Rocks:
These rocks are formed by the accumulation of rock fragments produced by weathering. These are settling down and harden into rock and lay down in layers.These may be shown in decorative stones. Turquoise grows mainly as vein in sedimentary rocks. Opal also occurs in sedimentary rocks.
Metamorphic Rocks:
Igneous or Sedimentary rocks that have been changed by heat and pressure within the earth, forms a new rock with new minerals. These rocks are called Metamorphic Rocks. In this reaction gemstones can grow within these rocks. In mica schists, garnets are grown in this way, which were once mudstones and clays. Marble, forms from lime stone may contain rubies due to high temperatures and pressure.
Organic Gems:
These are coming from plants and animals. Pearls are formed around foreign bodies that have made their way inside the shells of shellfish. Coral is made up of the skeletons of tiny marine animals. These animals are called coral polyps. Amber is fossilized tree resin, which is collected from sediments or sea. Jet is fossilized wood that can be found in sedimentary rocks.
Some gems such as quartz and garnet are found worldwide. More unusual geological conditions are necessary for formation of diamonds and emeralds and therefore they are rarer.
Gemstones are found in many parts of the world, singly or grouped together. Groups large enough to be worked are called deposits. There are two types of deposits. They are primary deposits and secondary deposits.
Primary Deposits:
Which gemstones are still at the first location (At the place of original formation) is called primary deposits. In these deposits gemstones still have their original relationship with their host rock. The crystals are usually well preserved but yield of the deposit is not very large and many tons of non-gem bearing rocks are mixed with the gemstones.
Secondary Deposits:
In secondary deposits, the gemstones have been transported from the place of their formation and deposited somewhere else. During this process, harder crystals become rounded and smaller. According to the way the gemstones have been transported, or according to the place of deposit, they can named as fluvial (river), Marine (in the sea), littoral (Coastal), or Aeolian (by the wind) deposits. Gem deposits which were deposited by river are called alluvial deposits. Between the primary and secondary deposits there are places where weathering or decomposition takes place. The gemstones are often found at the foot of high mountains, and have collected in such places as part of the weathered debris, because the specifically lighter host rock was carried away by rain and wind, the heavier gemstones were left. These are called eluvial deposits.
Distribution of gemstones over the earth is irregular and some regions are more favored than others. South and South East Asia, The Urals, South Africa, Brazil, Australia and the mountainous zones of the USA are the main regions which were the gemstones are found.