The purity of gold alloys is expressed in karats, (UK: carats) which indicates the ratio of the minimum amount of gold (by mass) over 24 parts total. 24 karat gold is fine gold (24/24 parts), and the engineering standard[要出典] is that it be applied to alloys that have been refined to 99.9% or better purity ("3 nines fine"). There are, however, places in the world that allow the claim of 24kt. to alloys with as little as 99.0% gold ("2 nines fine" or "point nine-nine fine).[要出典] An alloy which is 14 parts gold to 10 parts alloy is 14 karat gold, 18 parts gold to 6 parts alloy is 18 karat, etc. This is becoming more commonly[要出典] and more precisely expressed as a decimal fraction, i.e.: 14/24 equals .585 (rounded off), and 18/24 is .750 ("seven-fifty fine").
There are hundreds of possible alloys and mixtures possible, but in general the addition of silver will color gold green, and the addition of copper will color it red. A mix of around 50/50 copper and silver gives the range of yellow gold alloys the public is accustomed to seeing in the marketplace.
