A ruby is a gemstone that's usually a deep red. You can also describe things that are this same red color as ruby, like Dorothy's ruby slippers.
Rubies are one of four types of precious stones — the others are diamonds, emeralds, and sapphires. A ruby can be any reddish shade from pink to dark red, and it's the official birthstone for people born in July. Less expensive rubies are often treated with high heat, which improves the color and clarity of the stone. Ruby comes from the Medieval Latin rubinus lapis, "red stone."