Opals are a type of gemstone known for their unique play of color, which is caused by the way light interacts with their internal structure. They are primarily composed of silica and water, and can be found in various colors, including white, black, and boulder opals. The most valuable opals display vibrant colors and patterns, making them highly sought after in jewelry. These gemstones are formed in volcanic rock and can be found in several locations around the world, including Australia, Ethiopia, and Mexico. Opals are often used in rings, necklaces, and earrings, and are considered the birthstone for the month of October.