Mineraloid

Non-crystalline mineral-like substance

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 amorphous glass 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.

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