Opal is a hydrated amorphous form of silica (silicon dioxide, SiO2). It is deposited at a relatively low temperature and may occur in the cavities of certain rocks, such as volcanic rhyolite. Opal is also found as a replacement mineral in wood, bone, and other organic materials.

The formation of opal begins with the weathering of silica-rich rocks, such as granite and sandstone. This releases dissolved silica into groundwater. When the groundwater comes into contact with organic material, such as wood or bone, the silica can replace the organic material and form opal.

Opal can also form in volcanic environments. When lava cools quickly, it can trap water and gases, which can then form opal. This type of opal is known as hydrophane opal, and it can change color when it is wet.

Opal is a beautiful and valuable gemstone. It is used in jewelry, carvings, and other decorative objects.