Opal
Opal is SiO2·nH2O, up to 20% water, with an IMA row but no Handbook sheet. GIA: hardness 5 to 6.5, SG 2.15. A 20.00 g piece above 11.11 g in water is not opal.
Opal is hydrated silica, SiO2·nH2O, with its own row on the September 2026 IMA list, status G, but no Handbook of Mineralogy sheet. The International Gem Society calls it amorphous. GIA gives hardness 5 to 6.5, refractive index 1.37 to 1.47 and specific gravity 2.15, with up to 20% water. Precious opal shows play-of-colour; common opal does not.
In short
- An IMA row, but no Handbook sheet. The September 2026 IMA-CNMNC Master List lists opal as SiO2·nH2O, status G, with the year printed as a question mark and the country as unknown. The Handbook of Mineralogy has no opal sheet: the opal.pdf address returned 404 on 12 October 2026. Its cristobalite sheet puts fibrous or microcrystalline material in quotation marks as “opal”.
- Not one structure. Curtis and co-authors, Minerals 9 (2019), 299 call opal a generic term for a group of amorphous and paracrystalline silica species, sorted by X-ray diffraction into opal-A, opal-CT and opal-C. Whether that makes opal a mineral is set out on is opal a mineral.
- Water, worked. GIA says opal holds up to 20% water; the International Gem Society gives 3% to 21%, usually 6% to 10% in gem opal. On standard atomic weights that is n = 0.10 to 0.89 in SiO2·nH2O, and 0.21 to 0.37 for gem opal. Written SiO2·H2O, opal would be 23.1% water, above all three published maxima.
- The 11.1 g ceiling. A 20.00 g piece of opal weighs at most 11.11 g in water on the USGS top figure of 2.25, and 11.03 g on GIA’s 2.23. Glass starts at 11.30 g on the gem society’s 2.30, and crystal quartz reads 12.45 g. Above the ceiling, it is not opal; below it, it may still be synthetic.
- Play-of-colour is spheres, not pigment. GIA says precious opal is built of sub-microscopic silica spheres stacked in a grid; spheres about 0.1 micron across give violet and about 0.2 micron give red. In common opal, the International Gem Society says, the spheres are random in size, shape and arrangement.
| Material | Source | Specific gravity | 20.00 g piece in water | Against the 11.11 g ceiling |
|---|---|---|---|---|
| Opal | GIA facts panel | 2.15 (+0.08, −0.90), so 1.25 to 2.23 | 4.00 to 11.03 g | Below |
| Opal | USGS gemstones list | 1.98 to 2.25 | 9.90 to 11.11 g | Sets the ceiling |
| Opal | Geoscience Australia | 2.09 | 10.43 g | Below |
| Black and white opal | IGS | 2.10 | 10.48 g | Below |
| Fire opal | IGS | About 1.95 | 9.74 g | Below |
| Porous opal, often from Ethiopia | IGS | 1.74 to 1.89 | 8.51 to 9.42 g | Below, and rises while wet |
| Synthetic opal | IGS | 1.8 to 1.9 | 8.89 to 9.47 g | Below: the ceiling cannot catch it |
| Cristobalite | Handbook sheet | 2.32 to 2.36 | 11.38 to 11.53 g | Above |
| Glass | IGS glass article | 2.30 to 4.50 | 11.30 to 15.56 g | Above: not opal |
| Chalcedony | IGS | 2.55 to 2.91 | 12.16 to 13.13 g | Above |
| Quartz crystal | Handbook sheet | 2.65 | 12.45 g | Above |
What is opal? Hydrated silica with an IMA row and no Handbook sheet
The list. The September 2026 IMA-CNMNC Master List has a row for opal: formula SiO2·nH2O, status G for grandfathered, the year printed as a question mark, the country as unknown, the first reference as original paper? and a second reference of American Mineralogist 107 (2022), 1353. A script over the list finds two rows written exactly SiO2·nH2O, opal and mogánite. Hyalite, hydrophane, potch, opal-A, opal-CT and opal-C have no rows; they are names within opal, not species.
The Handbook. There is no Handbook of Mineralogy sheet for opal; the address opal.pdf, tried in both spellings, returned 404 on 12 October 2026. The word does appear on the cristobalite sheet, in quotation marks: cristobalite occurs fibrous or microcrystalline (“opal”), massive, and “opal” is among its associates. The quotation marks are the Handbook’s own; this page reads them as a label for material, not a claim of species.
Mineraloid. Opal is sometimes described as a mineraloid. None of the sources fetched for this page, GIA, the USGS, Geoscience Australia, the gem society or the 2019 review, uses that word, so this page does not classify opal by it. GIA’s facts panel gives Mineral: Hydrated Silica; Geoscience Australia calls opal a gemstone and a mineral. The question is worked on is opal a mineral.
How much water is in opal? SiO2·nH2O, worked
The published ranges. GIA says opal contains up to 20% water trapped in its silica structure. The International Gem Society gives 3% to 21% in its text and 1% to 21% in its properties panel, usually 6% to 10% in precious opal; both figures are printed here because the article prints both. The 2019 review says up to 20% water, as molecular H2O or silanol or both.
Worked. On standard atomic weights SiO2 is 60.083 and H2O 18.015. For a water mass fraction w, n = (w/18.015) ÷ ((100 − w)/60.083). At 3% water n is 0.10; at 6%, 0.21; at 10%, 0.37; at 20%, 0.83; at 21%, 0.89. A formula written SiO2·H2O, n = 1, would be 23.1% water, more than GIA’s, the gem society’s or the review’s maximum. In every published range read here, opal carries less than one water per silica.
Why it matters on a shelf. The gem society says opals are sensitive to changes in temperature and can craze, developing cracks as they dehydrate, and that Virgin Valley, Nevada opal has a strong tendency to crack from water loss in air. This page gives no storage or drying recipe; it records what the source says.
Precious or common opal: what play-of-colour is
GIA’s two classes. GIA divides opal into precious, which shows play-of-colour, and common, which does not. Play-of-colour comes from sub-microscopic silica spheres stacked in a grid, like Ping-Pong balls in a box; light diffracts between them. Spheres about 0.1 micron across give violet, about 0.2 micron give red. Geoscience Australia adds that larger spheres give all colours and smaller ones only blues and greens.
The types GIA names: white or light opal; black opal, with a dark bodycolour; fire opal, transparent to translucent with a brown, yellow, orange or red bodycolour, often without play-of-colour and also called Mexican opal; boulder opal, cut with its ironstone matrix; and crystal or water opal, transparent with a clear background. The gem society describes hydrophane opal, which turns transparent and shows colour when soaked in water or oil.
Assembled stones. Geoscience Australia describes opal doublets, a slice usually over 2 mm thick glued to a black base, and triplets, a slice about 0.5 mm thick between a black backing and a dome of glass or clear quartz. A triplet is three materials; neither a scale nor a hardness test describes it. See how to tell if a crystal is fake.
Is my opal real? The 11.1 g ceiling and where it fails
The 11.1 g ceiling: a 20.00 g piece that weighs more than 11.11 g in water is not opal on any published range read here. The top figures are GIA 2.23 (2.15 plus 0.08), the gem society’s maximum 2.23 and the USGS 2.25, worked as 20 − 20/SG to 11.03 g and 11.11 g. Glass, at 2.30 to 4.50 on the gem society’s glass figures, reads 11.30 g and up; the gem society says glass imitations usually have higher specific gravity and refractive index than natural opal, and contain bubbles and swirl marks. Method: how to measure specific gravity at home.
Where it fails, documented. Synthetic opal, at 1.8 to 1.9 on the gem society’s figures, reads 8.89 to 9.47 g and sits under the ceiling; so does opal impregnated with black plastic, which the same source says reads too low, and smoke-impregnated opal may float until it absorbs water. Porous opal, 1.74 to 1.89, absorbs water during the test and its reading rises, so the gem society says to weigh it immediately. The ceiling rules materials out; it never proves natural opal.
Hardness is no shortcut. GIA gives 5 to 6.5, so opal is softer than quartz at 7, and a scratch test damages a cut stone; see how to test mineral hardness properly. The separation from chalcedony, which is quartz, is on is chalcedony quartz and chalcedony.
Where opal is found: Australia, Ethiopia, Mexico and Nevada
Australia. Geoscience Australia says Australia produces 95% of the world’s precious opal, found in Cretaceous sandstones and mudstones where silica-bearing groundwater hardened into a gel in veins. Opal was first mined commercially at Listowel Downs, Queensland, in 1875; in 1915 a 14-year-old boy found opal at what became Coober Pedy; Lightning Ridge, New South Wales, is known for black opal; Queensland boulder opal sits in ironstone of hematite and goethite. The “opal pineapples” of White Cliffs are opal pseudomorphs after ikaite, which has its own IMA row, Ca(CO3)(H2O)6.
Elsewhere. The gem society names Virgin Valley, Humboldt County, Nevada, discovered around 1900, where opal fills cracks and seams in opalised wood; Zimapan, Hidalgo, Mexico, for yellow and red fire opal; Querétaro and San Luis Potosí; and Ethiopia, with discoveries of precious opal in 1994 and 2008, including near Wegel Tena. The 2019 review found its samples from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey were all opal-CT.
On this site. Silica relatives are quartz and chalcedony; the species pages are listed in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.