Igneous rocks : : These are rocks, which have solidified from the molten rocks, which lies molten deep beneath the earth's surface. 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.
Sedimentary rocks : : These rocks are formed by accumulation of rock fragments produced by weathering. In time these fragments settle down and harden into rock once again. Sedimentary rocks are normally found in layers and sometimes the gems found here show these layers in them.
Metamorphic rocks : : Metamorphic rocks are either Igneous or Sedimentary, which have changed beneath the earth due to heat and pressure and has resulted in new form of minerals in it. During this process gemstones also grow within them.