Although the weight of gold and silver is sometimes quoted in kilograms or grams, the official weight measurement to gauge the mass of precious metals is the troy ounce. One troy ounce (oz) equals 31.1034768 grams. Note that a troy ounce is not the same as an avoirdupois ounce (more commonly known as the kitchen ounce) which equals approximately 28.34 grams.
The purity of gold and silver is measured in millesimal fineness, which denotes the purity of the metal in parts per thousand. For example, the PAMP 100 gram gold bar contains a gold fineness of 999.9, which means that the bar is 99.99% pure gold. Another example, the $20 Double Eagle with a gross weight of 33.43624 grams contains a gold fineness of 900, which denotes a gold purity of 90% or 33.43 x 0.9 = 30.092 grams of pure gold.
In addition to fineness, the traditional purity measurement of gold is the karat (K). The highest karat rating is 24 and this equals or is greater than 99.9% pure gold. Gold jewellery in Europe most often contains a gold purity of 14 and 18 karats, which equals 14/24 = 58.3% and 18/24 = 75% pure gold respectively.