* Mineraloids are naturally occurring solid substances with a defined chemical composition, but they lack a specific crystalline structure like true minerals.
* Opal is formed when silica (silicon dioxide) dissolves in water and then precipitates out as an amorphous gel. It doesn't have a specific crystal structure, which is why it's classified as a mineraloid.
While opal can be found in sedimentary, igneous, and metamorphic rocks, it is not formed from those rocks themselves. It forms within them as a result of silica-rich fluids moving through the rock and depositing the silica.
