Australian Opal Types Compared with Ethiopian Opal: A Gemmologist's Guide

What are the main Australian opal types?

Opal is hydrated silica, written SiO₂·nH₂O. Unlike most gems it has no crystal structure. Precious opal is built from tiny silica spheres, roughly 150–300 nanometres across, stacked in tidy rows. Light bends around those rows and splits into spectral colours, an effect called play-of-colour. Opal without that order is called potch (common opal), which has body colour but no flashes. Our page on how opal's chemistry compares with diamond, sapphire and other gems explains why water and silica make it so different.

In Australia, opal is sorted into types by two things: how dark the body is, and whether it sits in its host rock. The four main Australian opal types are:

• Black opal: a dark body tone (N1–N4) that makes the colours glow. Mainly from Lightning Ridge, New South Wales.
• Boulder opal: thin veins of opal naturally attached to brown ironstone. From western Queensland.
• Crystal opal: transparent to semi-transparent, with play-of-colour that seems to float inside the stone. Any field, any body tone.
• White or light opal: a milky, light body tone (N7–N9). Mainly from Coober Pedy and other South Australian fields.

You will also hear dark opal (N5–N6, between black and white) and matrix opal (opal spread through its host rock rather than in a seam). Opal is Australia's national gemstone, and it sits alongside sapphire and chrysoprase among the coloured gemstones we cover. Any word here that is new to you is in our glossary of jewellery terms.

How is opal graded: body tone, brightness and pattern?

There is no single international opal grade like the 4Cs for diamonds. In Australia, the trade uses a body tone scale from N1 (darkest) to N9 (lightest), set out in the Australian opal nomenclature used by industry groups. Body tone is judged face-up, against a reference chart, ignoring the flashes of colour.

The dark body tone that defines black opal matters because a dark background makes the same play-of-colour look stronger, just as a torch looks brighter at night. After body tone, a buyer looks at five other things:

Black, boulder, crystal and white opal: what is the difference?

Black opal is the type most collectors chase. It comes almost entirely from the Lightning Ridge area of New South Wales, where it forms as nobbies (rounded lumps) in clay. Our guide to black opal from Lightning Ridge covers its grading and why a bright red-on-black stone is so rare.

Boulder opal forms in cracks and hollows inside ironstone concretions in western Queensland, around Winton, Quilpie, Yowah and Koroit. Because the seams are often paper-thin, cutters leave the ironstone on the back as a natural support. That makes boulder opal a natural, solid stone, not a doublet, and its free-form shapes are one of its charms. See Queensland boulder opal for the Yowah nut and matrix varieties.

Crystal opal is defined by transparency, not by field. A light crystal opal is clear with colour suspended inside; a black crystal opal is dark but still lets light through. The crystal opal guide explains how transparency changes the look.

White opal, also called light or milk opal, is the classic South Australian type from Coober Pedy, Andamooka and Mintabie. It is the most widely available Australian opal and usually the most affordable. Read more about white or light opal, and see where each Australian opal field is and what it produces.

What is Ethiopian opal?

Ethiopia became a major opal source in two waves. Opal from the Shewa (Mezezo) area appeared in the 1990s, much of it chocolate-brown and some later smoked to look black. Then in 2008, large deposits were found near Wegel Tena in the Wollo province, now sold as Welo opal. Welo opal has bright play-of-colour, often on a white, cream or clear body, and it is now widely sold around the world.

The key geological difference is where it forms. Australian opal is sedimentary: it formed in sandstone and clay basins as silica-rich water seeped into gaps. Ethiopian opal is volcanic: it formed in layers of rhyolite, a volcanic rock. That origin is linked to the most important practical difference.

Most Welo opal is hydrophane. Hydrophane opal is porous and absorbs water. When wet it can turn more transparent and its play-of-colour may fade or vanish, then return as the stone dries over hours or days. Porous opal can also take up oils, lotions and dirt, which may stain it permanently. Most Australian precious opal is non-hydrophane and does not change in water.

Ethiopia also produces some orange and red body-colour opal. That belongs to the separate family of orange fire opal, best known from Mexico. In Australia, older trade talk sometimes calls any opal with play-of-colour fire opal, so ask which meaning a seller intends.

How do Australian opal types compare with Ethiopian opal?

To the eye, a fine Welo opal and a fine light Australian crystal can look very similar. The differences show up over years of wear: how the stone reacts to water, hand cream and swimming, and how its value is seen. Neither is a better gem in principle. Australian opal has a long track record of stability; Ethiopian opal gives strong colour for the money, provided you accept extra care.

How can you tell Australian opal from Ethiopian opal?

Origin cannot always be proven from a single stone, even in a laboratory, but a gemmologist has several clues. These are the checks we use at the bench:

• Water response: hydrophane opal often feels slightly sticky on the tongue and darkens or clears when a drop of water sits on it. Australian precious opal usually does not. We do not recommend testing a stone you do not own.
• Specific gravity and refractive index: both are low for opal, about 1.98–2.25 for SG and 1.37–1.47 for RI. Welo opal often reads at the lower end of each range.
• Host rock: brown ironstone on the back points strongly to Queensland boulder opal. Sandstone or clay potch points to an Australian sedimentary field.
• Body and pattern: Welo opal often shows bright, three-dimensional colour with a slightly honey-toned or clear body. Very dark stones from Ethiopia should raise the question of smoking or dyeing.
• Paperwork: a seller should state the origin and any treatment in writing.

Treatments and imitations matter more than origin for most buyers. Smoked opal is Ethiopian opal heated in smoke so carbon darkens its surface to imitate black opal. Sugar-acid treated matrix opal, often from Andamooka, has carbon deposited in its pores to darken it. Lab-created opal (synthetic opal) shows a regular, columnar pattern often described as lizard-skin or chicken-wire under magnification.

Doublets are a thin slice of opal glued to a dark backing such as potch, ironstone or glass. Triplets add a clear dome of quartz or glass on top. Both are honest products when disclosed, and much cheaper than solid opal, but they should never be sold as solid.

Is opal durable enough for an engagement ring?

Opal is soft and brittle by gem standards. At 5.5–6.5 on the Mohs scale it scratches on everyday dust, which contains quartz at 7. Its toughness (resistance to chipping and breaking) is also poor, and sharp temperature changes can cause crazing, a network of fine surface cracks. Hydrophane opal adds the risk of absorbing water and skin products.

Opal can still work as an engagement ring if you choose the setting carefully and accept that it may need re-polishing over the years. A full bezel (a metal rim around the edge) or a raised edge of metal protects the girdle better than thin claws. Solid boulder opal, with its ironstone backing, is often a little more robust than a thin solid cut. Our guide to opal engagement ring wear and care covers daily habits in detail.

If you want an Australian stone that copes with daily knocks, compare Australian sapphire engagement rings or the green of chrysoprase, often called Australian jade. Opal also shares its month with a harder gem: see October birthstones, opal and tourmaline.

How much does Australian and Ethiopian opal cost?

Opal prices vary more than almost any other gem, because brightness, pattern and colour can multiply value many times over within one type. Prices are usually quoted per carat for solid opal. The ranges below are broad guides for gem-quality solid stones in Australia, not quotes.

Within each type, a bright red flash on a dark body, a broad pattern and colour that shows from every angle push prices to the top of the range. Faults such as potch lines, sand spots or crazing pull them down. An Ethiopian opal can look as bright as an Australian stone that costs several times more, which is the main reason it has become so popular. Natural opal is a gem to enjoy wearing, not a financial asset, so choose for the look you love.

How do you care for Australian and Ethiopian opal?

• Clean gently: a soft cloth and lukewarm water for solid Australian opal; just a dry or barely damp cloth for Ethiopian hydrophane opal.
• Never use ultrasonic or steam cleaners on any opal. Vibration and heat can crack it.
• Keep doublets and triplets dry: soaking can weaken the glue and cloud the join.
• Avoid sudden temperature changes: a hot car to a cold pool can trigger crazing.
• Keep oils and lotions away from Welo opal: porous stones can absorb them and stain.
• Store separately: harder gems and metal clasps will scratch opal.

Earrings and pendants take far fewer knocks than rings, so they are an easy way to wear opal. Choose secure earring backs for opal studs, and see our guide to chain styles for opal pendants. Opal also suits opal cufflinks and other men's pieces, which are rarely knocked in daily wear.

How do you choose between Australian and Ethiopian opal? Step by step

1. Decide how it will be worn. Daily ring wear calls for a solid, stable stone in a protective setting; earrings and pendants allow more choice.
2. Ask the origin and type in writing. Australian black, boulder, crystal or white, or Ethiopian, and whether it is solid, a doublet or a triplet.
3. Ask about treatment. Smoking, dyeing or sugar-acid treatment must be disclosed, as must lab-created opal.
4. Check body tone and brightness. Compare the stone against an N1–N9 chart and judge how vivid the colours are.
5. Rock the stone. Watch whether the colour shows from many angles or only one, and look for potch lines and crazing.
6. Look from the side. Check for a join line or glass cap.
7. Plan the setting and care. Choose a bezel or protective design, and accept that hydrophane opal needs to be kept dry.