Australia's national gemstones are opals, which are a type of mineraloid composed of hydrated silicon dioxide. Opals are known for their unique play of color, which is caused by the diffraction of light from the tiny silica spheres that make up the stone. Opals come in a variety of colors, including white, black, blue, and pink. They are found in many parts of Australia, and have been mined there since the late 19th century.