Though not required by law, most gold jewelry pieces carry a quality mark that denotes the proportion of pure gold each contains. The letter "K" in the mark stands for the word "karat," the unit used in the gold-content rating system.
Pure gold is defined as being 24K, or 24 parts gold with no other metal added. As gold of this purity is too soft for jewelry applications, it is often mixed with base metals to make it harder and more durable. Most commonly available pieces range from 22K to 10K or a gold content of 92 percent to 42 percent.