Opal, or hydrated silicon dioxide (SiO2 - nH2O), is a mineraloid that is often placed in the Quartz Group. It is composed primarily of silicon dioxide and varying amounts of water (the water usually makes up 5-10% of opal's composition). Mineraloids are an unofficial class containing materials that are found naturally and often treated as gemstones though they are actually not minerals, as they do not have a crystalline structure. Other examples of mineraloids are amber, pearl, jet, and obsidian. The Quartz Group is composed of gems that are forms of SiO2, also known as silicon dioxide or silica. There are no less than nine various ways that silicon dioxide can be organized into a neutral structure. The Quartz Group can also be called the Silica Group. Opal and lechatelierite are the two mineraloids included in the Quartz Group.

Opal can come in many...