Opal is a hydrated amorphous form of silica (SiO₂·nH₂O) that is prized for its unique play of color, known as opalescence. This phenomenon occurs when light is refracted through the internal structure of microscopic silica spheres, creating flashes of vibrant color, often in shades of blue, green, red, yellow, and orange. Opal can range from transparent to opaque and is often found in sedimentary rock formations. It has a Mohs hardness of 5.5 to 6, making it somewhat soft and prone to cracking if not handled carefully. Major sources of opal include Australia, Ethiopia, Mexico, and Brazil. Opal is widely used in fine jewelry, such as rings, necklaces, and earrings, due to its captivating appearance.