Carat (purity) for gold & platinum
The carat (abbreviation ct or K) is a measure of the purity of gold and platinum alloys. In the United States and Canada, the spelling karat is now solely used for the measure of purity, while carat solely refers to the measure of mass weight.
24 karat (millesimal fineness 999)
22 karat (millesimal fineness 916)
20 karat (millesimal fineness 833)
18 karat (millesimal fineness 750)
15 karat (millesimal fineness 625)
14 karat (millesimal fineness 585)
10 karat (millesimal fineness 417)
9 karat (millesimal fineness 375)

18 carat is therefore ¹⁸/₂₄ or ³/₄ gold. In recent years it has become common to refer to alloys millesimally, that is in “parts per thousand”. Eighteen carat gold is therefore 750 parts per thousand.

Troy
A system historically used for measuring the weight of precious metals and gemstones. The name comes from the French city of Troyes. 480 grains = 1 ounce troy