For the formation of corundum, the special geological environment is necessary, in which there is an excess of aluminum with simultaneous poverty of silicon, otherwise, as is usually the case, no corundum but aluminosilicates such as kyanite and andalusite are formed. Corundum is aluminum oxide. Its color is caused by trace elements.
The fact that blue corundum, i.e. sapphire, is formed owes this anomaly to other elements: Iron and titanium. These must also be available at the same time and place. This proves the extreme rarity of sapphires in the earth’s crust. The same can be said of differently colored sapphires, which require certain combinations of elements that occur in the right dose with aluminum and little silicon. For blue sapphire is the iron and titanium in the right combination.
Sapphires can sometimes be microscopically assigned to a country of origin. This is done by using the presence or absence, quantity and combination as well as shape of minerals, twin lamellae, rutile dust arrangement and color distribution. Likewise, healing cracks, which are the remnants of melting processes in corundum during genesis, can provide information about genesis, origin, and subsequent human-induced changes based on their appearance. More details about the stones afford laboratories.