Crystal Harvest Field Guide

Opal: The Complete Guide

Opal is the only popular gem that isn’t a crystal. Its flashes of color come from microscopic spheres of silica stacked like oranges. Here is the history, the science, and how to buy and care for it.

Quick facts

SiO2·nH2OHydrated silica, typically about 3–10% water by weight, sometimes more.
Not a crystalOpal is amorphous, a “mineraloid” rather than a true mineral.
5.5–6.5 MohsSofter than quartz, with toughness GIA rates “very poor to fair.”
SG ~2.15RI about 1.45, lower in some Mexican opal (as low as 1.37).
150–300 nm spheresPlay-of-color comes from light diffracting through orderly stacks of tiny silica spheres.
Sphere size = colorSmaller spheres flash blues and greens. Larger spheres can show the full spectrum, including red.
OctoberOpal is the traditional October birthstone.
Found on MarsNASA’s Mars Reconnaissance Orbiter detected opaline silica on Mars in 2008.

A short history of opal

Opal went from a Roman treasure to “bad luck” in a Victorian novel, and back to Australia’s national gemstone.

  1. NameThe name traces to Sanskrit upala, “precious stone,” which became Latin opalus.
  2. c. 250 BC onwardOpal reaches Roman trade, and Pliny the Elder records how highly Romans prized it. A well-known story says the senator Nonius chose exile rather than sell his opal to Mark Antony.
  3. 1597The first written record of opal mining at Dubník, near Červenica in today’s Slovakia. The idea that Roman opal came from these mines is local tradition without archaeological evidence.
  4. 1775The Harlequin opal (594 g) is found in the Slovak mines. It is now in Vienna’s Natural History Museum.
  5. 1829Sir Walter Scott’s novel Anne of Geierstein features a cursed opal. European opal sales reportedly fell sharply, though the often-quoted “50%” figure is hard to verify.
  6. 1870s–1915Australian discoveries arrive: Listowel Downs, Queensland (commercial mining by 1875), White Cliffs (1889), Lightning Ridge (first mined 1902), and Coober Pedy, found on 1 February 1915 by teenager Willie Hutchison.
  7. 1922Mining in Slovakia ends, unable to compete with Australia.
  8. 1926The 2,585-carat Roebling opal from Virgin Valley, Nevada, goes to the Smithsonian.
  9. 1956The Olympic Australis, about 17,000 carats, is found near Coober Pedy and named for the Melbourne Olympics.
  10. 1964–1970sJ. V. Sanders uses electron microscopy to reveal the sphere structure behind play-of-color. That knowledge leads to lab-grown opal, commercially produced by Gilson from about 1974–75.
  11. 1993Opal is proclaimed Australia’s national gemstone on 23 July.
  12. 2008A major deposit is found at Wegel Tena in Ethiopia’s Wollo (Welo) province. The same year, NASA reports opal on Mars.

How opal forms

Opal forms at low temperature from silica dissolved in water. Where that happens gives us two main families of opal.

  1. Silica dissolves. Rainwater and groundwater carry dissolved silica down through weathering rock.
  2. Water collects. The water pools against an impermeable layer or fills cracks and pores.
  3. Gel forms. As water evaporates in dry periods, the silica concentrates into a gel.
  4. Spheres settle. In rare cases the silica forms uniform spheres that settle into an orderly grid. That grid is what creates play-of-color.
  5. Sedimentary opal. In Australia, this happened in weathered Cretaceous sandstones and mudstones of the Great Artesian Basin.
  6. Volcanic opal. In Ethiopia and Mexico, opal fills pores and cavities in volcanic rock like rhyolite and ignimbrite.

Where opal is found

Australia dominates precious opal, but Ethiopia and Mexico have transformed the market.

Australia

NSW, SA, QLD

About 95% of the world’s precious opal. Black opal from Lightning Ridge, white opal from Coober Pedy and Andamooka, and boulder opal from Queensland.

Ethiopia

Welo (Wollo)

Found in 2008 in an opal-bearing volcanic layer up to a meter thick. Mostly white, crystal, and fire opal, generally cheaper than Australian opal of similar quality.

Mexico

Querétaro & Jalisco

Famous for fire opal in rhyolite. Opal cut together with its host rock is sold as “Cantera.”

Nevada, USA

Virgin Valley

Home of the Roebling opal. Material from here is known to craze.

Slovakia

Dubník

Europe’s historic source, mined until 1922.

Mars

Valles Marineris

Opaline silica detected from orbit in 2008, suggesting water lasted until perhaps 2 billion years ago.

Record breakers

  • Olympic Australis: about 17,000 carats (3,450 g), found in 1956 near Coober Pedy and long described as the largest and most valuable gem opal ever found.
  • Harlequin opal: 594 g, found in 1775 in Slovakia, now in Vienna’s Natural History Museum.
  • Roebling opal: 2,585 carats of black opal from Nevada, at the Smithsonian since 1926.

Types of opal

Black opal

Play-of-color on a dark body. The rarest type, mostly from Lightning Ridge.

White opal

Light body color, the classic Coober Pedy opal.

Crystal opal

Transparent to semitransparent, with play-of-color inside.

Boulder opal

Opal still attached to its ironstone host rock.

Fire opal

Yellow, orange, or red body color, often with no play-of-color. Read fire opal: named for its color.

Common opal

Opal without play-of-color, including milk opal, hyalite, and wood opal.

Hydrophane

Porous opal that absorbs water and temporarily turns clear. Much Welo opal behaves this way.

Doublets and triplets

Thin slices of opal glued to a backing, with triplets adding a clear cap. Sold for a fraction of the price of solid opal.

Treated, synthetic, and assembled opal

Opal is porous and valuable, which makes it a target for treatments. Here are the main ones.

PropertyWhat it isHow to spot it
Lab-grown (Gilson and others)Synthetic opal produced commercially since the mid-1970sA “lizard skin” or columnar pattern in the color patches under a loupe
Smoke treatmentEthiopian opal darkened with soot to mimic black opalBlack staining concentrated in surface pits and scratches. Natural dark opal is evenly dark throughout.
Sugar-acid treatmentPorous opal (mostly Andamooka matrix opal) soaked in sugar then acid to deposit black carbonTiny black specks concentrated in pores and cracks
Dyed or impregnatedPorous opal dyed or filled with oil, wax, or plasticFillers can be invisible and may need FTIR or Raman lab testing
Doublets and tripletsOpal slice glued to a backing, sometimes with a quartz or glass capLook at the side for a straight glue line

Myths vs. facts

Myth: Opal is bad luck.

Fact: The idea spread after Walter Scott’s 1829 novel, which was fiction.

Myth: Opal is a crystal.

Fact: It is an amorphous mineraloid, silica with water and no crystal structure.

Myth: Opal’s colors are pigments.

Fact: Play-of-color is light diffracting through microscopic silica spheres.

Myth: All fire opal flashes color.

Fact: Fire opal is named for its body color and often has no play-of-color.

Myth: An Ethiopian opal that goes clear in water is ruined.

Fact: That is hydrophane behavior, and it reverses as the stone dries.

Myth: Black opal is always natural.

Fact: Smoke, sugar, and dye treatments can imitate it.

Myth: Doublets and triplets are solid opal.

Fact: They are glued assemblies and should be sold as such.

Myth: Opal exists only on Earth.

Fact: NASA has detected opaline silica on Mars.

Care and display

Cleaning

Warm soapy water and a soft cloth only. No ultrasonic or steam cleaners.

Heat and light

High heat, sudden temperature changes, or strong lights can cause crazing (fine cracks).

Hydrophane opal

Don’t soak Welo opal. It can absorb 5–15% of its weight in water, clouding or clearing temporarily and risking cracks.

Doublets and triplets

Never soak them, since water can weaken the glue.

Settings

With moderate hardness and poor toughness, opal is safest in protective settings for rings.

Chemicals

Avoid hydrofluoric acid and caustic alkalis.

Opal in the shop

Hand holding a gem box containing an Ethiopian fire opal cabochon flashing green, blue and orange
Ethiopian Fire Opal Cabochon — Natural Play-of-Color$48.72
See all opal

Frequently asked questions

Can opal get wet?

Solid Australian opal can handle water. Don’t soak hydrophane Ethiopian opal or doublets and triplets.

Why is Ethiopian opal cheaper?

Ethiopia’s deposits are large and productive, so Welo opal generally costs less than Australian opal of similar quality.

What month is opal the birthstone for?

October.

How can I tell lab-grown opal?

Look under magnification for a “lizard skin” or columnar pattern in the color patches.

Which opal is the rarest?

Black opal, with vivid color on a dark body.

Is opal good for an everyday ring?

Only with care. It is moderately hard (about 5.5–6.5) with poor to fair toughness, so a protective setting is best.

Who should not wear opal?

There is no physical reason anyone cannot wear opal, but it is a delicate gem: about 5.5 to 6.5 on the Mohs scale, sensitive to sudden temperature changes, and it contains water. People who work with their hands, garden, or lift weights may prefer opal in earrings or pendants. The old superstition that opal is unlucky (unless you were born in October) largely traces back to Sir Walter Scott’s 1829 novel Anne of Geierstein, which hurt opal sales for decades.

Is opal an expensive stone?

It can be anything from inexpensive to extremely valuable. Common opal (no play-of-color) is cheap. Precious opal is priced by its play-of-color brightness, pattern, body tone, and origin; top black opal from Lightning Ridge, Australia, is among the most expensive gems per carat, while Ethiopian opal offers bright color at much lower prices.

What are the different types of opal?

The main types are: black opal (dark body tone, mostly Lightning Ridge, Australia); white or light opal; crystal opal (transparent to translucent); boulder opal (opal in ironstone, Queensland); fire opal (orange-red body color, mainly Mexico); Ethiopian (Welo) opal, which is often hydrophane and absorbs water; and common opal, which has no play-of-color.

How is opal formed?

Silica-rich water seeps into cracks and cavities in rock, then slowly evaporates, leaving behind microscopic spheres of silica. When those spheres are uniform in size and stack in an orderly grid, they diffract light into the flashes of spectral color called play-of-color. Disordered spheres make common opal.

What causes play-of-color in opal?

Diffraction. The orderly rows of silica spheres act like a diffraction grating, splitting white light into colors. Larger spheres can produce reds and oranges; smaller ones produce only blues and greens, which is why red play-of-color is rarer and more valuable.

What is opal good for?

Mainly jewelry and collecting. Opal is one of the October birthstones and is the national gemstone of Australia, which produces most of the world’s precious opal. It is also widely used in crystal-healing practices, though there is no scientific evidence for those effects. See our opal specimens.

Keep reading

Sources and further reading

Hardness, water content, and the date of the first synthetic opal vary slightly between sources. This page uses ranges where they disagree.

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