Flexi Says: Although opal is generally described as a mineral for historical reasons, it is not a single specific mineral because it can be composed of different minerals (cristobalite and/or tridymite) or amorphous silica. Its basic chemical composition is hydrous silicon dioxide, or SiO2 · nH2O. The "n" in the formula means that it can include a variable number of water molecules. Like other forms of silica minerals, such as quartz, opals can contain many different impurities which can produce a range of colors. Due to the many variations in opal composition, there are a great many varieties of opals some of which may be unique to single deposits. For example, the Hudson Institute of Mineralogy lists 70 opal varieties and this list is not exhaustive.