The measure of the fineness or purity of gold alloys is expressed as its karat. In the United States this is abbreviated as K, Kt, k, or kt. In Great Britain it is spelled carat, abbreviated as c or ct. This should not be confused with the entirely different system also called carat, which measures precious gemstone weights.
Using the karat system, gold’s purity is measured in twenty-four parts. 24k gold is therefore twenty-four/twenty-fourths gold, 100 % pure fine gold. In reality, pure gold is actually only 99.99% pure.
The international system used in most other countries expresses fineness of precious metals in parts per thousand. For this reason 24 karat gold will sometimes be marked 1000 or 999. Pure gold is much too soft for daily wear; almost all jewelry is less than 1000, 999, or 24k.
18k gold is eighteen parts gold and six parts other metals added for strength. This is 75% pure, sometimes marked as 750. It is slightly less yellow than 24k. The two most commonly used metals in the remaining metal composition of gold alloys are copper and silver.
Most gold jewelry in the U.S. is 14k gold- fourteen/twenty-fourths gold. 14k is 58.333% pure gold. Industry has converted this into the easier to manage and slightly finer 58.5% gold, sometimes marked 585. 14k is paler yellow than 18k gold, but it is more durable for jewelry use.
10k gold is ten/twenty-fourths gold. It is the lowest legal standard in the U.S.
Sterling silver is an alloy that must contain 92.5% pure silver. The other 7.5% is most often copper, but can be any metal or alloy and still be called sterling silver. It is usually marked 925 or sterling. Fine silver is too soft for most jewelry, but if used will be marked 999 rather than 925.