Opal rewards curiosity. Turn a good stone in your hand and the colour shifts from green to blue to a red that seems to come from somewhere deeper than the surface. That movement is not a trick of the cutting; it is the gem's internal structure doing something no other gemstone does.
For a beginner, that beauty is also the problem. Almost every opal looks impressive in a photograph, and sellers describe them in a vocabulary that assumes you already know the answers. Three qualities decide most of an opal's value: play-of-colour, brightness and pattern. Learn to judge those, and the rest of the jargon becomes much easier to handle.
Play-of-colour: the light that moves
Play-of-colour is the rainbow-like flash that appears, shifts and vanishes as you tilt the stone. It happens because opal is built from tiny spheres of silica arranged in a regular grid, which bends light and splits it into spectral colours. The physics matters less than the practical test: play-of-colour is directional. It shows itself at certain angles and disappears at others.
Some opals seem to have colour sitting on the surface, visible from almost any angle. Others flash only when you catch them just right. Neither is automatically better, but you need to know which one you are looking at. A stone that performs at one precise angle can look flat and grey on a moving hand.
It also helps to separate play-of-colour from body colour. Some Mexican fire opal is a vivid orange or red with little or no play-of-colour at all, and is valued for that body colour alone. Potch is the industry word for common opal with no play-of-colour whatsoever — perfectly attractive in its own right, but it should be sold as what it is.
Body tone
Underneath the flashes is the base shade: white, grey, black, or the orange of fire opal. Australian traders often grade body tone on a numbered scale running from N1 (darkest) to N9 (lightest), so a "black opal" is simply one with a dark body tone. A dark base tends to make the colours sitting on top look richer and more dramatic, which is a large part of why black opal draws so much attention.
Brightness: how far the colour carries
Brightness, sometimes called brilliance, describes how strongly the stone throws colour back at you. Brightness is commonly graded from B1 to B5, with B1 the brightest. A very bright opal looks lit from within, even in shade. A dull one can have lovely colours that only appear under a strong desk lamp.
Test brightness by holding the stone at arm's length in daylight and comparing it with another opal under the same light. Intensity reads clearly from a distance; small pattern details do not. And be sceptical of a jeweller's spotlight, which flatters every stone in the tray.
Pattern: the shape of the flash
Pattern is the arrangement of colour across the face of the stone — the size, shape and evenness of the flashes. Dealers use a set of traditional names for the common arrangements:
- Harlequin — broad, clearly separated squares or diamonds, like a mosaic. Rare and much sought after.
- Broad flash — large, sweeping areas of colour that move together. Very forgiving in jewellery.
- Rolling flash — a band of colour that travels across the stone as you turn it.
- Pinfire — many small, tight points of colour, like a scatter of sparks.
- Flagstone — angular, interlocking blocks, often seen in boulder opal.
- Chaff or straw — narrow, streaky flashes. These can still be bright, but usually fetch less than a crisp, even pattern.
Judge pattern across the whole face, edges included. A stone with a spectacular patch in one corner and flat grey everywhere else is not a bargain; it is a stone with one good corner.
The words sellers use
A short translation table for the terms that come up most often:
- Solid — a natural opal, cut and polished, with nothing added.
- Doublet — a thin slice of opal glued to a backing. Fine from above; the join can be visible from the side.
- Triplet — a thin slice with a backing and a clear quartz or glass cap. Cheaper, and generally less durable.
- Directional — colour only appears from particular angles.
- Boulder or matrix — opal naturally mixed with ironstone, where the dark host rock can make thin seams of colour look stronger.
None of these is a dirty word. They are simply different products at different prices, and a seller who explains which one you are being offered is a seller worth staying with.
How to look at an opal properly
- Ask to see it out of the case, ideally near a window. Shop lighting is designed to sell.
- Tilt it slowly through a full arc, not just side to side, and note where the colour appears and fades.
- Check the whole face: centre, edges, and the area under the dome where the cut is thinnest.
- Hold it at arm's length to judge brightness, then bring it close for pattern.
- Turn it over. Look at the base and sides, and ask directly whether it is solid, a doublet or a triplet.
- Place it against your skin, or in the setting you have in mind. Colours read differently against metal and skin.
- Ask about the returns policy before paying. Opal is difficult to assess in five minutes.
Buying your first opal without overthinking it
Rank the three qualities for yourself. Brightness is the easiest to see and the hardest to fake, so it is a sensible first filter. After that, look for an even pattern, then for rarer colour combinations — red flashes on a dark body tone are the classic prize, while blue and green are far more common.
Think about wear as well as beauty. Opal is softer than quartz and can craze or crack if it is heated or cooled suddenly, so it suits pendants and earrings more readily than a ring worn daily. Keep it away from ultrasonic cleaners, prolonged soaking and hot cars. A slow, steady change of temperature is kinder than a shock.
Tip: photographs and videos rarely capture directionality. If a seller's images show dazzling colour from every angle, look for a short video that shows the stone being tilted.
Finally, buy from someone with clear return terms. If you are spending a serious amount, ask whether an independent gem laboratory report is available, and consider having a qualified gemmologist or valuer look at the stone before you commit. You do not need to become an expert to enjoy opal — you just need to slow down, tilt the stone, and trust what your own eyes tell you.
Photo: PRISCILAFLORES / Pixabay
