Pure (24K) gold is too soft to be appropriate for everyday wear; therefore, gold jewelry is made of a mixture of gold and other metals (such as silver, copper, nickel and zinc). The K (karat) number tells us what fraction of 24 parts of the alloy are pure gold; ie., 14K is 14/24, or 58.33%, pure gold. The metals used in the alloy also determine the color of the gold; a greater percentage of nickel gives white gold its color, while a high percentage of copper lends a reddish tone to rose gold. While gold itself does not tarnish, these alloying metals can sometimes cause a piece to darken with time, or leave a dark residue on the skin. In Europe, gold is marked with a number indicating the gold content as parts per thousand; ie., 18K gold, which is 75% pure gold, is stamped 750.
Platinum is rarer, and therefore more expensive, than gold. It is grayish-white in color, non-tarnishing, and very strong.Very high temperatures are necessary to melt platinum; therefore, it did not become a viable jewelry metal until advancements in jewelers’ tools were made in the late 1800s. It became the most popular jewelry metal in America in the 1920s; its durability made it the ideal choice of the period. Jewelry platinum is an alloy, usually 90% platinum and 10% iridium. Platinum jewelry is typically stamped “Plat.”