Mineraloid

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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 derived from decaying wood under extreme pressure. Opal is another mineraloid because of its non-crystal nature. Pearls, considered by some to be a mineral because of 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.

[edit] Common mineraloids

• Amber, organic, non-crystal structure.
• Anthracite, a variety of coal, "hard coal."
• Coal, organic, nonhomogeneous, and non-crystal structure.
• Jet, not considered a true mineral due to organic, non-crystal nature.
• Lignite, a variety of coal, "Brown coal."
• Limonite, amorphous hydrated iron oxide.
• Obsidian, volcanic glass - non-crystal structure.
• Opal, non-crystal structure.
• Pearl, often considered a mineral due to crystal components, but then not a mineral due to organic origin.
• Petroleum, organic and liquid.
• Pyrobitumen, organic, nonhomogeous, non-crystal structure.
• Vulcanite, vulcanized natural or synthetic rubber, thus not a mineral due to organic composition, lack of crystal structure, and not naturally occurring.

[edit] See also

• List of minerals - Mineraloids are listed after minerals in each alphabetically sorted section.