Mineraloid · Silica
Opal
SiO2·nH2O
Opal is hydrated amorphous silica; precious opal from Australia (Lightning Ridge black opal, Coober Pedy) and Ethiopia (Welo) shows play-of-colour from ordered silica spheres, fire opal from Mexico is orange, and common opal and hyalite are colourless or milky. It is a mineraloid, not an IMA mineral, but collectors treat it as one; opalised fossils (Australian belemnites and shells) bridge both hobbies. Opal is soft, heat-sensitive and can craze.
| Formula | SiO2·nH2O |
|---|---|
| IMA status | mineraloid |
| Crystal system | Amorphous |
| Mohs hardness | 5.5–6.5 |
| Group | Silica |
| Type locality | None |
Fakes and treatments
Doublets and triplets (thin opal on a dark backing under a quartz cap) are sold as solid opal; Ethiopian opal is often smoke-treated to darken it or dyed; synthetic Gilson opal shows a 'lizard skin' pattern under magnification.
Elsewhere
- Wikidata: Q171058
- Wikipedia: opal
- Mindat: Opal
Cite this article
Stratum Study. "Opal" (Mineral). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/minerals/opal. Accessed 2026-09-20.
@misc{stratum-opal,
title = {Opal},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/minerals/opal}},
year = {2026},
note = {Mineral article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.