Mineraloid
Definition
A mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals.
Example
Opal and obsidian are common mineraloids because they lack an ordered crystal structure.
The Almost-Minerals
In geology, a mineral must meet five specific criteria: it must be naturally occurring, inorganic, solid, have a definite chemical composition, and—crucially—have an ordered <strong>crystalline structure</strong>. A "mineraloid" breaks this last rule. Its atoms are not arranged in a neat, repeating lattice but are jumbled together randomly (amorphous).
Famous Mineraloids
Some of the most beautiful gemstones are actually mineraloids:
- Opal: Hydrated silica spheres packed together without a crystal lattice.
- Obsidian: Volcanic glass that cooled too fast for crystals to form.
- Amber: Fossilized organic resin (also breaks the "inorganic" rule).
- Moldavite: Glass formed by meteorite impacts.