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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.
Other naturally occurring substances that appear superficially to be minerals in nature are Nonminerals.
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