Gemstones that have a mineral origin are found in rock , 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 Metermorphic is a continues process, best described in terms of the rock cycle. Gem quality minerals with in these rocks may be easily accessible at the Earth surface or lie buried deep beneath it. Others, Separated from their host rock by erosion, are carried by rivers to lakes or the sea.
Igneous rocks have solidified from molten rock, which comes from deep beneath the Erath’s surface. Some, called ”extrusive” Igneous rocks, are thrown out from volcanoes as lava, volcanic bombs or ash. ”Intrusive” Igneous rocks are those that solidify beneath the surface. Essentially, the slower a rock cools and solidifies, thelarger the crystals and therefore the gemstones formed within it. Many large gemstone crystals from in a kind of intrusive Igneous rock known as pegmatite.
Metamorphic rocks are either igneous or sedimentary rock that have been changed by heat and pressure within the Earth to from new rocks with new minerals. As this happens, gemstones can grow within them. Garnets for example form in rock called mica schists, which were once mudstones and clays. Marble formed from limestone that has been subjected to intense pressure and high temperatures may contain rubies.
Sedimentary rocks are formed by the accumulation of rock fragmens settle down and harden in to rock once more. Sedimentary rocks are usually laid down in layers and these may be shown as a future in decorative stones. Most Australian opal occurs mainly as veins in sedimentary roch such as shale, halite and gypsum are sedimentary rocks.