Mineraloid
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 a dense form of
coal.
Opal is another mineraloid due to its non-crystal nature.
Pearls, considered by some to be a mineral due to the presence of calcium carbonate crystals within their 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.
A very common mineraloid is
limonite a mixture of hydrated iron oxides.
Other naturally occurring substances that appear superficially to be minerals in nature are
Nonminerals.
*
List of minerals Mineraloids are listed alphabetically after minerals.
