Gemstones that have a mineral origin are found in rocks, or in gem gravels derived from these rocks. Rocks themselves are made up of one or more minerals, and may be divided into three main types. The formation of these three types- igneous, sedimentary, or metamorphic is a continuous process, best described in terms of the rock cycle. Gem quality minerals within these rocks may be easily accessible at the Earth's surface, or lie buried deep beneath it. Others, separated from their host rock by erosion, are carried by rivers to lakes or the sea.

Essentialy, the slower a rock cools and solidifies, the larger the crystals - and therefore the gems found within it. Many large gemstone crystals are found in a kind of intrusive igneous rock known as pegmatite. Some GEM MINERALS, such as quartz and garnet, are found worldwide. Others like diamonds and emeralds, are rarer due to the more unusual geological conditions necessary for their formation. Even when a mineral is found worldwide, only a minute proportion may be of gem quality. The main gem location of the world are therefore those where gem quality material occurs in sufficient quantity to make production economical.