mineraloid
Non-crystalline rock material
A mineraloid is a naturally occurring, mineral-like substance that lacks the crystalline structure required to qualify as a true mineral.
Obsidian, opal, and amber are mineraloids—they form through geological processes but have amorphous or organic compositions that set them apart from minerals proper.
The distinction matters mainly to geologists and gemologists.
Hydrated silica gemstone
- amberHardened ancient tree secretion
- jetForced fluid stream
- pearlMollusk-formed gemstone
- moldaviteMeteorite impact glass
obsidian · tektite · fire opal · black opal · white opal · ammolite · cabochon · polished
Glassy igneous rock
- opalHydrated silica gemstone
- pumiceLightweight volcanic abrasive
- perliteHeat-popped volcanic glass
- pitchstoneGlassy volcanic rock
scoria · lechatelierite · fulgurite · tektite · moldavite · volcanic glass
opal
Hydrated silica gemstone
moldavite · obsidian · tektite · fire opal · black opal · white opal · ammolite · cabochon · polished · lapidary
obsidian
Glassy igneous rock
pitchstone · scoria · lechatelierite · fulgurite · tektite · moldavite · volcanic glass
amber
Hardened ancient tree secretion
ammolite · nacre · ivory · coral · shell · bone · bitumen · asphalt · petroleum · peat