White gold doesn't exist in nature. It is an alloy of yellow gold and a mixture of nickel, palladium, rhodium, and other metals, which turns the yellow gold to white. Very common in jewelry, 18-karat white gold is often used in watchcases and bracelets, especially as a less expensive, similar-looking alternative to naturally white platinum. By weight white gold is commonly 75% gold, 3% silver, 12.5% palladium and 9.5% copper (although combinations of gold, silver, nickel, zinc and copper are used quite often). Although very expensive, the addition of palladium (or even less costly nickel and zinc) makes this gold alloy very hard compared to most other alloys. Since white gold has such a low percentage of copper content, it is the least likely gold alloy to cause an allergic reaction. Moreover, 18-karat white gold polishes well, resists scratching, and is one of the densest gold alloys.