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.
Obsidian and pumice are igneous rocks that solidified so rapidly from a melt that their atoms were unable to move into an ordered atomic structure. Instead, they rapidly formed a random network of atoms known as a “glass.” Obsidian and pumice are amorphous, and their compositions can vary dramatically from one location to another and from one volcanic eruption to the next. Obsidian and pumice are also mineraloids.
