What is the primary component of opal?

The primary component of opal is silica, specifically in the form of hydrated silicon dioxide. Unlike crystalline forms of silica, which are structured and repeat in a regular pattern, opal has a more disordered, non-crystalline structure. This unique arrangement allows opal to display its characteristic play-of-color, which is a result of the way light interacts with the microstructures within the stone.

Water is indeed a significant part of opal's composition, typically making up 5% to 10% of its weight, but it is not the primary component. Ironstone is a type of sedimentary rock that may contain iron minerals, and while it can be associated with the environments where opals are found, it does not constitute the composition of opal itself. Quartz is another form of silica, but it differs in that it is crystalline and does not share the same characteristics or structure as opal. Therefore, silica is the fundamental building block that defines opal's unique properties.