Mineraloid
From Wikinfo
A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that lie outside the generally accepted ranges for specific minerals. For example, obsidian is a glass and not a crystal. Jet is a dense form of coal.
References
- Adapted from the Wikipedia article, "Mineraloid" http://en.wikipedia.org/wiki/Mineraloid, used under the GNU Free Documentation License
