Emery is a dark, extremely hard rock used for grinding, smoothing, and polishing. When crushed into powder or glued onto boards and cloth, emery works like super-tough sandpaper that can shape metal, sharpen tools, and polish gemstones.

You've probably seen emery boards: those rough nail files people use to shape their fingernails. The gritty surface is coated with tiny particles of emery that gradually wear away the nail to smooth it. Before modern manufacturing, craftspeople relied on emery to sharpen knives, polish jewelry, and finish metalwork that needed to be perfectly smooth.

Emery gets its incredible hardness from a mineral called corundum, the same substance that forms rubies and sapphires. While gemstone-quality corundum becomes precious jewelry, lower-grade corundum mixed with other minerals becomes emery, a workhorse material that has shaped and polished human creations for thousands of years.

The word can also describe a dark gray color, like the color of emery itself. Emery might not be as glamorous as the gemstones it's related to, but it's the gritty, practical cousin that gets the real work done.