A
mineraloid is a
mineral-like substance that does not demonstrate
crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example,
obsidian is an
amorphous glass and not a
crystal.
Jet is derived from decaying wood under extreme pressure.
Opal is another mineraloid because of its non-crystalline nature.
Pearl, considered by some to be a mineral because of the presence of
calcium carbonate crystals within its structure, would be better considered a mineraloid because the crystals are bonded by an organic material, and there is no definite proportion of the components.
