How is a ruby formed in nature?
Ruby is a red variety of the mineral corundum. Corundum is formed in igneous rocks that are relatively high in aluminum, but very low in silicon and in metamorphic environments with similar chemistry. In order to get rubies, there must also be chromium present in the rocks. By definition, rubies get their color from a little bit of chromium contamination in the crystal structure. So, if the red color is derived from the presence of some other element, then the stone is not a ruby; it is a sapphire. (A sapphire is any color of corundum that is not a ruby.