Opals are a type of opaque gemstone that is composed of hydrated amorphous silica. They are usually found in sedimentary rocks and can range in color from white to yellow, orange, pink, red, green, blue, and purple. Opals are known for their unique play of color, which is caused by the diffraction of light through the stone's microstructure. They are often used in jewelry and are considered to be a symbol of hope, purity, and innocence.