The chemical symbol for gold comes from a Latin word meaning "shining dawn." It has become synonymous with avarice. Pure gold is for kings, but jewelers prefer to work with alloys of gold -- gold mixed with other metals.
From a jeweler's point of view, gold has some problems. Mostly, it is too soft -- which means a jeweler's finished product may not hold its shape. Pure gold bends easily and scratches easily. The solution is to alloy gold with a little of some other metal so the alloy would be easy to work but the finished product would be hard and scratch-resistant.
In most countries, it is illegal to falsely hallmark a metal object. To convert hallmarks to percentages, put a decimal point between the second and third digits of the hallmark and read off the percent. Therefore, a hallmark of 925 means the objec is 92.5 percent gold and 7.5 percent another metal.
In the karat system, 24 karats means pure gold. Half gold would be 12 karats and 3/4 gold would be 18 karats. For any system like this, calculate part/whole = hallmark. (The first three digits past the decimal point are the hall mark.) A 15 karat item should be hallmarked 625 because 15/24 = 0.625.