Opal is not made from a specific chemical formula like most minerals. It's a hydrated amorphous form of silica. This means:

* Silica (SiO2): Opal is primarily composed of silicon dioxide, the same compound found in quartz.

* Water (H2O): Water molecules are trapped within the silica structure, giving opal its unique properties. The water content can vary, influencing the opal's color and play-of-color.

So, the formula for opal isn't a precise chemical equation, but rather a description of its composition: SiO2·nH2O, where 'n' represents the variable amount of water molecules.