18 karat gold is 75% pure gold alloyed with 25% other metals such as copper, silver, or zinc. This combination creates a durable metal that holds its color well and resists scratching. 18K is the global standard for fine jewelry and luxury watches — brands like Cartier, Rolex, and Tiffany predominantly use 18K gold. It offers the best balance of purity, durability, and value for high-end pieces.