Gold, in its natural form, is soft, dense, and inert. It also has a yellow sheen that is rather garish by today’s discreet luxury standards. In the watch industry, gold is often mixed with other metals to make it stronger and more resistant, creating what are called gold alloys. Mixing also changes the colour of gold; depending on the types and amounts of metal added, alloys can range from red-gold to brilliant white.
The measurement unit used for the purity of gold is the karat (k). Gold that is labelled 18k, for instance, is composed of 75% gold and 25% other metals. Gold that is certified 24k is pure gold without any metals added. Note that the higher the karat, the less hardy the gold is, and vice versa.