Carat (purity)
The karat or carat is a unit of purity for gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

Karat purity is measured as 24 times the purity by mass:

is the karat rating of the material,
is the mass of pure gold or platinum in the material, and
is the total mass of the material.

Therefore, 24-Karat gold is fine (99.9% Au w/w), 18-Karat gold is 18 parts gold 6 parts another metal (forming an alloy), 12-Karat gold is 12 parts gold (12 parts another metal), and so forth.

In England, the Karat was divisible into four grains, and the grain was divisible into four quarts.