Opal is the one gemstone that rewards a slow look. Turn it in your hand and the colours move, shift and sometimes disappear altogether, which is why two stones of the same size can sit at wildly different prices. Buying well comes down to a handful of things you can learn in an afternoon: body tone, brightness, pattern, and knowing whether you are looking at a solid stone or an assembled one.

Body tone: the first thing to judge

Body tone is the darkness of the stone itself, not the colours flashing inside it. The trade uses a scale running from N1, which is jet black, to N9, which is milky white. Black opal sits at N1 to N4, dark opal at N5 to N6, and light opal from N7 upwards.

A dark body makes the play of colour look sharper and richer. A single red flash on black reads far louder than the same flash on a pale crystal. That said, dark is not automatically better: a light crystal opal with a tight, brilliant pattern can outshine a dull grey-black stone every time. Judge body tone in daylight, not under the shop lamp, which flatters almost everything.

Brightness: what you are actually paying for

Brightness is how strongly the colours read across the face of the stone. Dealers describe it as faint, dull, moderate, bright or brilliant, and some use a numbered scale where the lower number is the brighter stone. The practical test is distance. Hold the opal at arm's length. If the colours still read clearly from there, it is a bright stone. If you have to bring it close to your eye to see anything, it is not.

Then take it outside. Look at it in open daylight, in shade, and in a dull corner of the room. Opals are chameleons and a good seller will not mind you doing this. Note the worst light you see it in, because that is the light you will often wear it in.

Patterns worth paying for

Pattern is the shape the colour makes, and it is where personal taste matters most. A few you will hear named:

• Harlequin — a mosaic of angular patches of roughly equal size, usually the most sought-after pattern.
• Flagstone — irregular flat blocks, often with good colour separation.
• Pinfire — hundreds of tiny sharp points of colour.
• Flashfire — broad, sweeping flashes that roll across the stone as you move it.
• Ribbon or Chinese writing — fine, hieroglyph-like strokes of colour.

Tightly packed, sharply defined patterns generally command more than scattered, mossy ones. Red on black is the classic combination at the top of the market; green and blue are far more common. Multi-coloured stones that shift through the spectrum are especially prized. Whatever the pattern, check that the brightness is even across the whole face. Dead patches where no colour shows will drag the value down.

The main types of opal you will see

Black, white and crystal

Black opal comes mostly from Lightning Ridge in New South Wales and is the most valuable solid type. White opal, largely from Coober Pedy in South Australia, has a milky body and is usually the most affordable solid opal. Crystal opal is transparent to translucent, and can be either dark or light depending on the background.

Boulder opal

Queensland boulder opal forms naturally in thin veins through ironstone. The ironstone is part of the stone, so nothing is glued, and the result is a genuinely tough opal. Look for patterns that fill the vein attractively rather than sitting in a sea of brown rock.

Ethiopian and fire opal

Ethiopian opal is often spectacular value, but much of it is hydrophane: it absorbs water, can go cloudy or turn clear when wet, and may lose its flash temporarily. Treat it differently, which I cover below. Mexican fire opal is usually a vivid orange or red body, often faceted and sometimes with no play of colour at all. It is sold on body colour, so judge it by a different standard.

Doublets, triplets and treatments

A solid opal is a single natural stone. A doublet is a thin slice of opal glued onto a backing of ironstone, black potch or resin to give it a dark background. A triplet goes further: an even thinner slice of opal, a backing, and a clear quartz or glass cap domed over the top.

None of this is dishonest, and a good triplet can show a fine black opal pattern for a fraction of the cost of a solid stone. But a doublet or triplet is not an investment stone, the cap can scratch, and water can eventually work its way in at the edges. Solid stones are also treated: Andamooka matrix opal is often sugar-and-acid treated to darken it, and smoking, dyeing and fracture filling all exist. Synthetic and imitation opals, including glass and resin, are common in cheap jewellery.

Ask directly, and expect a straight answer:

1. Is this solid opal, a doublet or a triplet?
2. Has it been treated in any way?
3. Is it natural, synthetic or imitation?
4. Where was it mined?
5. Can I see it in daylight before deciding?
6. What is your return policy?

Checking the stone and the setting

Look closely for crazing, the fine network of surface cracks that signals internal stress, and for chips along the edges. Check for sand spots, cloudy patches and dead areas in the face. A jeweller's loupe helps, and so does ordinary daylight.

The setting matters as much as the stone. Opal is a relatively soft gemstone, around 5.5 to 6.5 on the Mohs scale, so it chips and scratches more easily than sapphire or diamond. For a ring, a bezel or rubover setting that protects the edges is far safer than exposed claws. Opals in pendants, earrings and brooches take far less punishment. On any UK precious metal setting, look for an assay office hallmark.

Looking after opals in the UK climate

Opal holds water within its structure, and it dislikes fast changes in humidity and temperature. That makes the British year a challenge in a particular way: damp outside for months, dry indoors the moment the central heating comes on in November.

• Store in a soft pouch inside a sealed bag or box, with a barely damp cotton wool pad alongside during dry spells.
• Keep away from radiators, airing cupboards, dehumidifiers and sunny windowsills.
• Wear it regularly, but remove it for showers, washing up, swimming, the gym and gardening.
• Never use an ultrasonic or steam cleaner, and avoid household cleaners.
• Clean with lukewarm water, a drop of mild washing-up liquid and a soft cloth or brush, then dry it straight away.
• Do not soak Ethiopian hydrophane opal, and do not oil it.
• Put jewellery on last when dressing, and take it off first at night.

If you spot fine cracks appearing, stop wearing the piece and take it to a lapidary or gemmologist for an opinion.

Buying with confidence

Buy the stone you love in the light you will actually wear it in, and choose a seller who answers questions plainly and offers a return window. If the sum involved matters to you financially, treat it like any other significant purchase: get the stone described in writing on the invoice, and consider an independent valuation for insurance. A solid opal with moderate brightness and a clean face will give you more pleasure over thirty years than a heavily treated stone bought for its photograph.

Photo: RDNE Stock project / Pexels