Black Opal Guide: Colour, Value, Treatments and Ring Care

What is black opal?

Black opal is precious opal with a dark body tone, meaning the background colour of the stone, which ranges from dark grey to jet black. Against that dark base, the flashes of spectral colour, called play-of-colour, look brighter and more saturated than they do in a pale stone. That contrast is the whole appeal, and it is why gem-quality black opal is the most valued member of the opal family. It is one of the coloured gemstones we cover in depth, and one of the few that Australia supplies to the world almost on its own.

Opal is hydrated silica, SiO₂·nH₂O, usually holding about 3–10% water by weight. Unlike sapphire or diamond, it has no crystal structure; it is amorphous (non-crystalline). Our page on how opal's chemistry compares with diamond, sapphire and other gems sets the formulas side by side.

Precious opal is built from microscopic silica spheres, roughly 150–350 nanometres across, stacked in an orderly grid. Light passing between them is diffracted, which means split into its separate colours, a little like the rainbow on the back of a CD. The size of the spheres decides which colours appear: smaller spheres give blue and violet, larger ones can produce orange and red. Because the larger spheres needed for red are less common, red play-of-colour is the rarest.

Opal without play-of-colour is called common opal or, in the Australian trade, potch. Black potch is often the natural backing layer of a Lightning Ridge stone, and it is what gives a solid black opal its dark base. Opal's refractive index is low, about 1.45, so it has a soft, glowing lustre rather than the sharp sparkle of a faceted gem. Its specific gravity is about 2.1, noticeably lighter in the hand than quartz. Any term here you have not met is explained in our plain-English jewellery glossary.

What colours does black opal come in?

Two separate things describe the colour of a black opal: the body tone underneath, and the play-of-colour on top. The Australian opal trade grades body tone on a scale from N1 (jet black) to N9 (very light), viewed face-up on the top of the stone, not through the back.

A stone at N5 or N6 is usually sold as dark or semi-black opal, and should be priced below a true N1–N3 stone with the same colours. At the pale end of the scale you are into white opal with its milky, light body tone. There is also black crystal opal: a dark stone that is translucent, so you can see colour from within as well as on the surface. Our guide to transparent and semi-transparent crystal opal explains how that changes the look.

Play-of-colour can show any spectral colour. In rough order of rarity, from most to least: red, orange, yellow, green, blue and violet. A stone showing red across most angles is far rarer than one showing blue and green. The arrangement of the colour, called the pattern, matters just as much:

• Harlequin: distinct, roughly square patches like a mosaic. The rarest and most sought-after pattern.
• Broad flash: large sheets of colour that switch on and off as the stone moves.
• Rolling flash: a band of colour that rolls across the face as you tilt it.
• Pinfire: many tiny points of colour; attractive but usually valued lower.
• Floral, ribbon and Chinese writing: named patterns with organic shapes, lines or script-like strokes.

Where does black opal come from?

Almost all the world's fine black opal comes from Lightning Ridge, in northern New South Wales near the Queensland border. Commercial mining there dates from the early 1900s. The opal forms as small nodules, known locally as nobbies, in a layer of claystone inside the sedimentary rocks of the Great Artesian Basin. Miners sink shafts or use open cuts to reach that layer, and most nobbies are either potch or show little colour; gem black opal is a small part of what comes out of the ground.

Opal was proclaimed Australia's national gemstone in 1993. Other Australian fields produce different opal: Coober Pedy and Andamooka in South Australia are known mainly for white and crystal opal, and Mintabie, also in South Australia, produced some dark and semi-black opal. Our guide to Australia's main opal fields, from Lightning Ridge to Coober Pedy covers each one.

Western Queensland produces Queensland boulder opal, which forms in ironstone. Its dark brown ironstone backing can make it look like black opal face-up, but it is a separate type and should be sold under its own name. Outside Australia, some natural dark opal has come from Ethiopia, Honduras and Mexico, but a lot of the dark Ethiopian opal on the market has been smoke-treated to darken it, which we cover further down.

How does black opal compare with other opal types?

Every opal type looks different and is valued differently. This table puts black opal beside the other main types you will see for sale.

The biggest practical difference for a wearer is stability. Most Lightning Ridge black opal is a sedimentary opal with a good record of staying stable once cut. Much Ethiopian Welo opal is hydrophane, meaning it soaks up water and can temporarily turn clear or lose colour when wet. Our comparison of Australian opal types against Ethiopian opal goes into this in detail. Mexican fire opal is different again: it is prized for its orange body colour, and play-of-colour is a bonus rather than the point.

What affects black opal value and price in Australia?

Opal is not graded on a single standard scale the way diamonds are, so every black opal is valued stone by stone. Dealers and valuers weigh these factors, roughly in this order:

• Body tone: N1–N2 is worth more than N4, and a dark opal at N5–N6 sits a clear step lower.
• Brightness: how intensely the colour glows. A dull stone with lots of colour is worth less than a bright one with less.
• Colour: red is the rarest, then orange; blue-green stones are the most common.
• Pattern: harlequin and broad flash rank highest; pinfire is usually lower.
• Directionality: a good stone shows colour from most angles. If it only lights up when tilted one way, it is directional and worth less.
• Faults: crazing (fine surface cracks), sand or potch lines, windows (dead patches with no colour) and inclusions all reduce value.
• Cut and size: a smooth, even dome, good thickness and a pleasing outline add value; carat weight multiplies everything.

Small dark specks of potch or ironstone in an opal are judged mainly on how they look and where they sit, which is quite different from how gemmologists assess black inclusions in a diamond. A speck near the edge of an opal may barely matter; one in the centre of the face is a fault.

These are broad ranges only; black opal prices vary more than almost any other gem, and two stones of the same weight can differ tenfold. Doublets and triplets cost a small fraction of a solid stone. Choose a black opal because you love how it looks, and ask for a written description of body tone, brightness and pattern so you know what you paid for.

Are black opals treated? Doublets, triplets and lab-created opal

A solid black opal is opal all the way through, sometimes with natural potch at the back. Because black opal is valuable and thin seams are common, the trade also makes composite stones, which are legitimate when they are disclosed:

Other treatments make lighter opal look black. Smoke treatment darkens porous opal, mainly Ethiopian, with carbon that sits near the surface, so a scratch or chip can show a paler colour beneath. Sugar-acid treatment deposits carbon in porous matrix opal, mainly from Andamooka. Dyeing and resin or polymer impregnation are also used. Any treatment should be disclosed when you buy.

Lab-created opal, also called synthetic opal, has been made since the 1970s; the best known is Gilson. It is real opal-type silica grown in a laboratory, and it must always be sold as lab-created or synthetic, never simply as opal. Under magnification it often shows a columnar or "lizard skin" pattern within each colour patch. Imitations such as glass or plastic opalite are not opal at all; plastic feels warm and noticeably light for its size.

Is black opal good for an engagement ring?

Black opal can be worn in a ring, but it needs more care than most gems. It is 5.5–6.5 on the Mohs scale, and ordinary household dust contains quartz at 7, so everyday wiping and contact slowly dull the polish. Its toughness, meaning resistance to chipping and cracking, is fair to poor, and opal can crack from a sharp knock or a sudden change in temperature. Here is how it compares with other ring stones:

If you want a purple or golden stone that copes with more wear, amethyst, a quartz at 7 on the Mohs scale, golden citrine and ametrine, the two-toned mix of both are all harder than opal. For a dark stone that will take daily knocks, black diamonds offer a far harder alternative, though they look completely different, with no play-of-colour.

If you have your heart set on a black opal ring, the setting does most of the protecting:

• Full bezel: a rim of metal around the whole edge. The most protective choice and the one most opal cutters recommend.
• Low profile: keep the stone close to the finger so it is less exposed to knocks.
• Halo: a ring of diamonds around the opal adds contrast and shields the girdle (the outer edge).
• Claws: possible for occasional-wear rings, but each claw must sit carefully so it does not put pressure on the opal.
• Pendants and earrings: take far less wear than rings, so they suit black opal very well.

Our guide to opal engagement rings and everyday wear covers settings, band styles and what to expect over the years in more detail.

How do you care for black opal jewellery?

Solid black opal from Lightning Ridge is generally stable, and a little care keeps it looking its best for decades:

• Cleaning: warm water, a drop of mild soap and a soft cloth or very soft brush. Rinse and dry straight away.
• Never use ultrasonic cleaners, steam cleaners, bleach or harsh chemicals; heat and vibration can crack opal.
• Doublets and triplets: wipe only with a damp cloth; never soak them, or the glue can fail and the stone can fog.
• Temperature: avoid sudden changes, such as hot water after a cold day, saunas and leaving jewellery on a sunny dashboard.
• Take it off for the gym, gardening, cleaning, swimming and anything with hard knocks.
• Storage: keep it separate in a soft pouch so harder gems and metal do not scratch it.
• Repolishing: a scratched or dull solid opal can usually be repolished by an opal cutter, losing only a tiny amount of weight.

Opal is also one of the two birthstones for October, which makes black opal a popular birthday gift. See our guide to opal and tourmaline as the October birthstones if you are weighing up the two.

How do you choose a black opal? Step by step

Whether you are buying a loose stone or a finished piece, the checks are the same. Ask whether the stone is natural or lab-created, solid or composite, and whether it has been treated, and expect a clear written answer.

1. Confirm what it is. Natural solid opal, doublet, triplet or lab-created opal, and whether any treatment has been used.
2. Check the body tone. Look at the face of the stone; true black opal is N1–N4. Ask for the N grade in writing.
3. Judge brightness and colour. View it in daylight and under a single lamp. Bright colour beats lots of dull colour.
4. Turn it through 360°. Make sure the colour shows from most angles and note the pattern.
5. Look for faults. Use a loupe to check for crazing, windows, sand and potch lines across the face.
6. Check the side profile. A straight, even join line suggests a doublet; a natural potch backing is irregular.
7. Plan the setting. Choose a bezel or protective halo for a ring, and decide how often it will be worn before you commit.