A ruby is a precious gemstone known for its deep red color. Rubies form deep underground over millions of years when the mineral corundum mixes with chromium, which gives them their distinctive crimson glow. The finest rubies gleam with a color sometimes called pigeon's blood red, a vivid, slightly purplish red that seems to glow from within.

Rubies rank among the hardest natural substances on Earth, second only to diamonds. This hardness, combined with their brilliant color, has made them treasured throughout history. Ancient warriors believed rubies granted protection in battle and wore them as talismans. Kings and queens have worn ruby crowns for thousands of years. Today, a flawless ruby of significant size can be worth more than a diamond of the same weight.

The word ruby also describes anything with that characteristic deep red color. You might see ruby slippers (like Dorothy's famous shoes in The Wizard of Oz), ruby grapefruit, or ruby lips in a poem. When someone celebrates their fortieth wedding anniversary, it's called a ruby anniversary because forty years of marriage is considered as precious and enduring as the gemstone itself.