Gem stones that are of mineral origin are found in the rocks, or in gem gravels derived from these rocks. Rocks are made up of one or more minerals. The formation of rocks is a continuous process. Gem minerals are easily accessible at the earth’s surface or lie buried deep in the earth crest. Sometimes these gem laden rock gravels get separated from these rocks due to erosion and are carried long distances by rivers to lakes, sea and shallow areas.
In Igneous rocks, which form due to solidification from the molten rocks, many gemstones are found. Some igneous rocks are extrusive where it solidifies after volcanic eruption and some are intrusive rocks where it solidifies beneath the earth. The rate of solidification determines the size of crystals found. Slower the rate, larger the crystal and consequently larger the size of gemstones in it.