FORMULA: Al2O3

MOHS: 9

Corundum is a fairly unassuming name for one of the most prized gemstones in the world. You probably know it by some other names, sapphires and rubies. What’s the difference between a sapphire and a ruby? Marketing. But there’s more to them than just pretty reds and blues. Corundums can sometimes display one of the rarest and most gorgeous properties in minerals – asterism.

Asterism is that shimmery, cats-eye effect seen in tiger’s eye stones and in star sapphires. It comes from inclusions of tiny rods of a mineral called rutile, which are thinner than human hair and grow in long, parallel sets through the stone. These rutile silks catch the light, bounce it around, and put literal stars in our eyes.