18K gold is an alloy made of 75% pure gold and 25% other metals, such as copper or silver. The "K" stands for karat, which is a measure of the purity of gold. This composition gives 18K gold a rich color and enhances its durability, making it suitable for various types of jewelry. Due to its high gold content, 18K gold is more expensive than lower karat options like 14K gold or 10K gold. It is often chosen for fine jewelry, including rings, necklaces, and bracelets, as it strikes a balance between beauty and strength.