Opal is hydrated silica, a form of silicon dioxide that holds water and has no crystal structure. Precious opal shows play-of-color: flashes of spectral color that shift as the stone moves. That effect comes from tiny silica spheres stacked in regular rows. Australia has been the dominant source of fine opal since the late nineteenth century, and opal is one of the two modern October birthstones.
What opal is
Opal is classed as a mineraloid because it is amorphous, with no regular crystal lattice. Its water content usually falls between about 3 and 10 percent by weight, and some opal holds more. It is softer than quartz, with a Mohs hardness of 5.5 to 6.5, a low refractive index around 1.45 and a specific gravity near 2.1.
Under an electron microscope, opal turns out to be made of silica spheres, each a fraction of a micrometer across. In common opal the spheres vary in size and sit in a jumble. In precious opal they are nearly the same size and packed in an orderly three-dimensional grid, like oranges stacked in a crate.
How play-of-color works
The gaps between the ordered spheres act as a diffraction grating. White light entering the stone is split into spectral colors, and the color you see at any angle depends on the size of the spheres and the spacing between them. Smaller spheres diffract blue and violet; larger spheres can also produce orange and red. Red play-of-color needs larger spheres in a well-ordered structure, which is one reason red flashes are rarer and more valued.
The ordered regions are usually small patches, so a stone shows a pattern of color patches that shift and flicker when tilted. Traders name the patterns: pinfire for tiny points, flash for large patches that appear and vanish, and harlequin for a mosaic of angular patches. Harlequin is among the rarest. Play-of-color is one of several optical phenomena described on the gemstone optics page.
Precious and common opal
Precious opal is any opal showing play-of-color. Common opal, sometimes called potch, lacks it. Common opal can still be attractive and is cut in colors such as pink, blue (Peru), green and yellow. Fire opal, discussed below, may or may not show play-of-color.
Types of opal
Opals are classified by body tone (the background color, from black to white) and transparency.
- Black opal has a dark body tone, from dark gray to black, which makes the play-of-color stand out. It is the most valued type. Lightning Ridge in New South Wales is its classic source.
- White or light opal has a pale body tone. It is the most common type of precious opal.
- Crystal opal is transparent to semi-transparent, with play-of-color visible inside the stone. It can have a light or dark body tone.
- Boulder opal forms as thin seams of precious opal in ironstone boulders in Queensland, Australia. Cutters leave the dark ironstone attached as a natural backing.
- Fire opal is transparent to translucent with a yellow, orange or red body color. Mexico is the main source. Many fire opals have no play-of-color and are faceted for their body color.
- Matrix opal has precious opal spread through the host rock. Some matrix opal from Andamooka is treated with sugar and acid to darken the background.
Australian opal fields
Opal in Australia formed in sedimentary rocks of the Great Artesian Basin. Over millions of years, silica dissolved by weathering seeped down through sandstone and clay and settled in cracks, cavities and even fossil shells and bones, where it slowly hardened into opal. The sedimentary rocks and gemstones page explains this kind of deposit.
Lightning Ridge
The Lightning Ridge field in New South Wales is famous for black opal, found as nodules called "nobbies." It began producing commercially in the early twentieth century.
Coober Pedy
Coober Pedy in South Australia has been a major source of white and crystal opal since opal was found there in 1915. The summer heat is extreme, and many residents live in underground dugouts.
Andamooka and others
Andamooka, also in South Australia, produces white and crystal opal and the matrix opal mentioned above. Mintabie in South Australia and the Queensland boulder opal fields around Winton and Quilpie are other historic sources.
Opal is the national gemstone of Australia.
Ethiopian opal and hydrophane
Since the late 2000s, the Welo (Wollo) region of Ethiopia has produced large amounts of opal from volcanic rocks. Welo opal is often white to crystal with bright play-of-color, and much of it is hydrophane: it absorbs water readily. When wet, hydrophane opal can turn transparent and lose its play-of-color for a time, which returns as it dries. Because it is porous, it can also soak up oils, lotions and dyes. Some Ethiopian opal has been smoked or dyed to imitate black opal.
Earlier, Ethiopia's Shewa province produced darker opal in nodules. Other sources of precious opal include Mexico, Brazil (Piaui), Honduras, and Nevada's Virgin Valley in the United States.
Doublets, triplets and synthetics
Thin slices of precious opal are often assembled into composites:
- A doublet is a thin layer of opal glued onto a dark backing such as ironstone, black glass or common opal, which deepens the color.
- A triplet adds a clear dome of quartz, glass or plastic on top to protect the thin opal slice and magnify the color.
Composites are much cheaper than solid opal and should be labeled. Look at the side of the stone for a straight glue line, and be cautious with stones set in closed-back mountings. Triplets and doublets should not be soaked in water, which can loosen the glue and cloud the layers.
Synthetic opal with play-of-color was first produced by Pierre Gilson in France in the 1970s. It often shows a "lizard skin" or columnar pattern under magnification. Plastic imitations and opalescent glass are also sold. The gemstone treatments explained page covers dyeing, smoking and sugar treatment.
Crazing, durability and care
Crazing is a network of fine cracks that can appear in some opal, usually as it loses water after mining. Some deposits are more prone to it than others. Miners and dealers sometimes rest opal for months before cutting to see whether it crazes. Australian opal is generally regarded as stable.
Opal's hardness of 5.5 to 6.5 means it scratches from dust and hard surfaces. It is also sensitive to knocks, heat and sudden temperature changes. Store it separately from harder gems and avoid leaving it in hot, dry places. Rings need a protective bezel setting and careful wear.
Clean opal with a soft cloth and lukewarm water with mild soap. Never use ultrasonic or steam cleaners. Do not soak hydrophane opal or assembled doublets and triplets. More guidance is in the jewelry cleaning guide and on gemstone durability.
History and birthstone
Romans prized opal highly, and Pliny the Elder described its shifting colors. A reputation for bad luck in nineteenth-century Europe is often traced to Walter Scott's 1829 novel Anne of Geierstein, in which an opal plays a role in a character's fate. Queen Victoria's fondness for opal helped revive its popularity. Opal also featured in Art Nouveau jewelry. Beliefs about opal's luck are folklore; see gemstone meanings and folklore.
Opal is the October birthstone, shared with tourmaline on the modern birthstones by month list. It is a traditional gift for the fourteenth wedding anniversary.
Frequently asked questions
What causes the colors in opal?
Precious opal contains tiny silica spheres stacked in regular rows. The spaces between them diffract light into spectral colors that change with viewing angle.
Which type of opal is worth the most?
Black opal from Lightning Ridge, with bright play-of-color including red, usually commands the highest prices.
Can opal get wet?
Solid opal can be rinsed briefly. Hydrophane opal and doublets or triplets should not be soaked, because water can change their appearance or loosen glue.
How do I tell a solid opal from a triplet?
Look at the side of the stone. A triplet shows straight layers and often a clear dome on top, while solid opal has no glue lines.