A mineraloid is a naturally occurring substance that resembles a
mineral, but does not demonstrate the
crystallinity of a mineral. Mineraloid substances possess
chemical compositions that vary beyond the generally accepted ranges for specific minerals, for example,
obsidian is an
amorphousglass and not a true
crystal; lignite (
Jet) is derived from the decay of wood under extreme pressure underground; and
opal is a mineraloid substance because of its non-crystalline nature.
Pearl is a mineraloid substance because the
calcite crystals and the
aragonite crystals are bonded by an organic material, and naturally occurs without definite proportions of the components.
The first usage of the term mineraloid substance was in 1909, by mineralogist and geologist Julian Niedzwiedzki, in identifying and describing amorphous substances that resemble minerals.[1]