Naturally occurring sapphires were formed about 150-200 million years ago, and they form in feldspathoid-bearing igneous rocks (Feldspathoids are a group of tectosilicate minerals that make up nearly 75% of the Earth’s crust) that are undersaturated with respect to silica. Sapphires can also form in recrystallized limestone and in high-grade metamorphic rocks that are poor in silica and rich in aluminum. Sri Lanka, Burma, U.S. (Montana), Tanzania, Australia, Thailand, Cambodia, and the Kashmir region of India/ Pakistan are all places where sapphires can be found around the world.
Naturally occurring sapphires are formed by volcanic processes deep below the surface and the high temperature and pressure conditions of metamorphic processes. As liquid magma cools, minerals dissolve into crystals. The purest forms of corundum (sapphires are a part of the corundum family) are created by recrystallization of minerals during the metamorphosis of rocks that are of igneous origin. Sapphires’ colors are formed by mineral impurities that seep into the aluminum oxide. This is called crystal field or a ligand-field effect. Blue sapphires get their color from iron and titanium.