Rubies are formed of a mineral called corundum, comprised of aluminum oxide. The red color is caused by traces of chromium. Corundum also forms sapphire in many colors, which generally come from ...

Chromium turns the corundum pink or red, and that's how we get rubies. What's more, whole other minerals – such as rutile (titanium dioxide) and zircon – can get trapped within sapphires as ...