I always get pretty confused with what a carat is. And so do most people! Ask them the difference between 24-carat gold and a 100-carat diamond and they look blank. And the reason for this confusion is that a carat is BOTH a measure of mass (weight) as well as a measure of purity.
1. The carat is a unit of mass equal to 200 mg and is used for defining gemstones and pearls. In diamonds, for example, a paragon is a flawless stone of at least 100 carats (20 g).
2. The carat is ALSO a measure of the purity of gold alloys, 24-carat being pure gold. Carat purity is measured as 24 times the purity by weight of a gold alloy:
X = 24*Mg/Mm
where X is the carat rating of the material,
Mg is the mass of pure gold or platinum in the material and
Mm is the total mass of the material.
Therefore, 24-carat gold is fine (99.9% w/w), 18-carat gold is 75% gold, 12-carat gold is 50% gold and so on. Hope this helps the next time you are planning to buy a diamond ring or a gold necklace for your loving wife or girlfriend.
Visual Courtesy: http://www.flickr.com/photos/paparutzi/