How are rubies formed?

Rubies form when metamorphic geological processes occur, where extreme heat and pressure naturally press aluminium together with oxygen so the mineral corundum can be made, and trace chromium later enters the stone and gives it that distinct red colour.

For a ruby to form in nature, corundum needs to crystallise where chromium is plentiful, but silica and iron are completely missing. That kind of elemental setting is exceptionally rare in the crust, which is why high-quality large rubies are so hard to find. You usually see this element mixed in marble deposits linked to the ancient collision of tectonic plates, which is the reason the rarity holds on.