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 red corundum, i.e. ruby, is formed owes this anomaly to yet another element: chromium, which itself is present in the earth’s crust in only 0.02 percent by weight, must also be present at the same time and place. This proves the extreme rarity of rubies in the earth’s crust.
Rubies 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.