Why Is Opal a Mineraloid?

Opal is a mineraloid because it is amorphous hydrated silica. Despite its mineral-like properties, opal's chemical structure is hydrated with varied water concentration. Because of this variance, the stone's structure isn't fixed or organized and thus, qualifies as a mineraloid.

Is opal a mineraloid?

Opal is a hydrated amorphous form of silica (SiO. nH₂O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals.

Why are the mineral opal is often referred to as a mineraloid?

There are a number of familiar materials that can be classified as mineraloids. For example, opal is an amorphous hydrated silica with a chemical composition of SiO₂^.nH₂O. The "n" in its formula indicates that the amount of water is variable. Therefore, opal is a mineraloid.