A karat is a measurement of the fineness of a piece of gold. The fineness of a piece represents the weight of fine metal that it contains in proportion to the total weight, including alloyed metals and any impurities. Diamonds and other gemstones are measured in carats per 200mg, meaning for every 200mg of diamond, the higher the carat.
Gold uses karats. With gold, a karat is a fractional measure of purity for gold alloys, measured per 24 parts whole. Pure gold that has not been mixed with any other metals is therefore the highest possible karat, 24 karat (24k) gold.
Whilst more valuable, pure gold is rarely used for jewelry pieces because of its softness and malleability, and tendency to scratch and mark. Therefore, it is usually alloyed with base metals for use in jewelry, altering its malleability, color and other properties.