Precious Opal
A variety of Opal
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About Precious Opal
Formula:
SiO2 · nH2O
Colour:
Multi-coloured
Lustre:
Vitreous, Sub-Vitreous, Greasy
Hardness:
5½ - 6½
Name:
The origin of the word "opal" is uncertain. It may be from the Sanskrit "upala", meaning "stone" or "precious stone" or from opalus, the ancient Latin name for the gem (Pliny the Elder, 75-79). Pliny may have also referred to the gem as paederos, but a modern commentary by Kostov (2008) questions if that name was actually applied to the opal in the modern sense.
A valuable gem-variety of opal.
Most precious opal is opal-AG. It shows a play of colours ("opalescence") that is due to diffraction of light from the regular packing of submicroscopic silica spheres of roughly equal size (Jones et al., 1964; Sanders, 1964). Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous" (Jones and Segnit, 1971).
Not all precious opal is opal-AG: The diffracting structure and the resulting opalescence may be preserved to some degree when opal-AG transforms to opal-CT by crystallization (Sanders, 1975).
It usually forms from low temperature deposition from groundwaters in cavities in sedimentary and igneous rocks.
Visit gemdat.org for gemological information about Precious Opal.
Most precious opal is opal-AG. It shows a play of colours ("opalescence") that is due to diffraction of light from the regular packing of submicroscopic silica spheres of roughly equal size (Jones et al., 1964; Sanders, 1964). Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous" (Jones and Segnit, 1971).
Not all precious opal is opal-AG: The diffracting structure and the resulting opalescence may be preserved to some degree when opal-AG transforms to opal-CT by crystallization (Sanders, 1975).
It usually forms from low temperature deposition from groundwaters in cavities in sedimentary and igneous rocks.
Visit gemdat.org for gemological information about Precious Opal.Unique Identifiers
Mindat ID:
6666 (as Precious Opal)
3004 (as Opal)
3004 (as Opal)
Long-form identifier:
mindat:1:1:6666:8 (as Precious Opal)
mindat:1:1:3004:1 (as Opal)
mindat:1:1:3004:1 (as Opal)
Pronunciation of Precious Opal
Pronunciation:
| Play | Recorded by | Country |
|---|---|---|
| Jolyon Ralph | United Kingdom |
Physical Properties of Precious Opal
Vitreous, Sub-Vitreous, Greasy
Transparency:
Translucent
Colour:
Multi-coloured
Streak:
White
Hardness:
5½ - 6½ on Mohs scale
Fracture:
Conchoidal
Chemistry of Precious Opal
Mindat Formula:
SiO2 · nH2O
Elements listed:
Synonyms of Precious Opal
Other Language Names for Precious Opal
German:Edelopal
Varieties of Precious Opal
| Bandfire Opal | A precious opal with bands showing a play of colours. |
| Black Opal | The most precious kind of opal, this variety of precious opal has a dark background colour. |
| Boulder Opal | A variety of Precious Opal found in Queensland, Australia, as cracks or coatings in and around ironstone/sandstone boulders. |
| Claro Opal | Transparent mexican Precious Opal with an intense play of colours (red, green, blue and yellow). |
| Contra Luz Opal | A precious opal where the play of colours is only visible when held up to the light. |
| Crystal Opal | A transparent to translucent variety of Precious Opal where the colour play is visible both on the surface and in the interior. |
| Harlequin Opal | A variety of Precious Opal in which the play of color is arranged in a vivid harlequin, diamond-shaped, or rectangular-shaped pattern. |
| Lechosos Opal | A variety with a milky-white background colour. |
| Levin Opal | Precious Opal with long thin lightning-like flashes. |
| Mother of Opal | A sandstone or ironstone with Opal as a cement. |
| Opal Matrix | A term for a thin layer of opal on a host rock. |
| Painter Boulder | Sandstone boulders with a thin coating of opal. |
| Pinfire Opal | Opal with very small pinhead-sized colour flashes. |
| Pyrophane | Opal where colour play appears to wander about at random. |
| Red Flash Opal | Precious Opal with red colour flashes that appear and disappear as the stone is turned. |
| White Opal | Precious Opal with a white background. |
| Yowah Nut | Small rounded pebbles of sandstone impregnated with Precious Opal. Originally reported from Yowah opal field, Queensland, Australia. |
Common Associates
Associations Based on Photo Data:
| 79 photos of Precious Opal associated with 'Petrified Wood' | |
| 30 photos of Precious Opal associated with Opal | SiO2 · nH2O |
| 22 photos of Precious Opal associated with 'Black Opal' | SiO2 · nH2O |
| 9 photos of Precious Opal associated with 'Boulder Opal' | SiO2 · nH2O |
| 6 photos of Precious Opal associated with 'Milk Opal' | SiO2 · nH2O |
| 4 photos of Precious Opal associated with 'Harlequin Opal' | |
| 3 photos of Precious Opal associated with 'Shell Opal' | |
| 3 photos of Precious Opal associated with 'Opal Matrix' | SiO2 · nH2O |
| 2 photos of Precious Opal associated with 'Fire Opal' | SiO2 · nH2O |
| 2 photos of Precious Opal associated with 'Bone Opal' | SiO2 · nH2O |
Other Information
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Precious Opal
mindat.org URL:
https://www.mindat.org/min-6666.html
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References for Precious Opal
Reference List:
Baier, Ernst (1932) Die Optik der Edelopale. Zeitschrift für Kristallographie, 81 (1-6). p.183-218. doi:10.1524/zkri.1932.81.1.183
JONES, J. B., SANDERS, J. V., SEGNIT, E. R. (1964) Structure of Opal. Nature, 204 (4962). 990-991 doi:10.1038/204990a0
DARRAGH, P. J., GASKIN, A. J., TERRELL, B. C., SANDERS, J. V. (1966) Origin of Precious Opal. Nature, 209 (5018). 13-16 doi:10.1038/209013a0
Jones, J. B., Segnit, E. R. (1971) The nature of opal I. nomenclature and constituent phases. Journal of the Geological Society of Australia, 18 (1) 57-68 doi:10.1080/00167617108728743
Sanders, J. V. (1975) Microstructure and crystallinity of gem opals. American Mineralogist, 60 (9-10) 749-757
Elzea, J.M., Odom, I.E., Miles, W.J. (1994) Distinguishing well ordered opal-CT and opal-C from high temperature cristobalite by x-ray diffraction. Analytica Chimica Acta, 286 (1). 107-116 doi:10.1016/0003-2670(94)80182-7
Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., Ostroumov, M. (2008) Common gem opal: An investigation of micro- to nano-structure. American Mineralogist, 93 (11) 1865-1873 doi:10.2138/am.2008.2518
Rondeau, Benjamin; Gauthier, Jean-Pierre; Mazzero, Francesco; Fritsch, Emmanuel; Bodeur, Yves; Chauvire, Boris (2013) On the Origin of Digit Patterns in Gem Opal. Gems & Gemology, 49 (3). p.138-146. doi:10.5741/gems.49.3.138
Liesegang, Moritz; Milke, Ralf (2018) Silica Colloid Ordering in a Dynamic Sedimentary Environment. Minerals, 8 (1). p.12. doi:10.3390/min8010012
Localities for Precious Opal
Showing 241 localities.
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The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
Yowah opal field, Paroo Shire, Queensland, Australia