Ruby is the red variety of corundum (Al2O3), one of the hardest natural minerals on Earth, ranking 9 on the Mohs hardness scale—second only to diamond.

Its vivid color comes from trace amounts of chromium, which replace a small fraction of aluminum atoms in the crystal lattice. Even a chromium concentration of less than 1% is enough to turn colorless corundum into a gem that fluoresces with an intense red glow under ultraviolet light. The most valued specimens display a saturated red often described as “pigeon blood” with slight blue undertones, a color balance created by precise chromium concentrations and minimal iron content.

These rubies were naturally crystallized over millions of years under extreme heat and pressure within metamorphic rocks such as marble. Ruby crystals grow in a hexagonal system and can withstand temperatures exceeding 2,000°C (3,600°F) before melting.

Unlike synthetic rubies—first used to create the world’s first working laser in 1960—natural rubies contain microscopic inclusions that record their geologic history, acting as tiny time capsules of the conditions deep within Earth’s crust.