Why Opals Reward a Careful Eye

Opal is the one gemstone that refuses to play by the usual rules. There is no universal cut grade, no single number that captures its beauty, and no two stones that behave quite the same way. When you are comparing a tray of loose opals, or weighing up a single set piece in a jeweller's window, what you are really judging is two things at once: a light show and a structural risk. Get both right and you will own something that stops people mid-sentence. Get it wrong and you may find that the stone dulls, cracks or disappoints within a year.

The good news is that a calm, methodical assessment takes only a few minutes once you know what to look for. Here is how to go about it.

Judging Play-of-Colour Properly

Play-of-colour is the spectral flash that shifts as you tilt the stone. Assess it in three separate ways rather than trusting a single glance.

• Brightness. Hold the stone at arm's length. If it still throws colour at that distance, it is genuinely bright. Stones that only come alive under a counter lamp, held six inches from your face, are usually dimmer than they first appear.
• Pattern. Look for definition. Large, broad, clearly separated flashes — often described as harlequin or flagstone — are rarer and more valuable than fine, pinfire speckle, though a well-distributed pinfire can be lovely in its own right.
• Coverage and directionality. Slowly tilt the stone through a full half-turn. A good opal flashes from several angles. If the colour only appears in one narrow position, think about how the stone will sit once set — you may spend most of your life looking at a flat, pale face.
• Colour range. Reds against a dark background remain the most sought after. Greens and blues are far more common. A single vivid colour usually beats a muddy mixture of several.

Always view opal in daylight near a window, not just under shop lighting. Fluorescent tubes can flatter a dull stone enormously.

Body Tone and Transparency

Body tone is the underlying colour of the stone beneath the play-of-colour. Dark opal is graded on a scale from N1 to N9, where N1 to N4 is genuinely black or dark, N5 to N6 is mid-grey, and N7 to N9 is light. A darker body generally makes the colour appear more dramatic, but tone alone does not guarantee a good stone — a bright N4 will outshine a murky N1.

Transparency matters just as much. Crystal opal is transparent to semi-transparent, and light passes through it, which can make the colour appear especially luminous. White or milky opal is opaque and tends to look softer and more diffuse. Crystal stones are beautiful, but be aware that they reveal internal inclusions and any backing more readily, so inspect them closely.

Crazing, Cracks and Long-Term Stability

Crazing is the fine network of surface and internal cracks that develops when an opal loses moisture. It can be present from the start or appear later, and it rarely improves. This is the single most important thing to check before you commit.

Use a 10x loupe under good light and look for:

• hairline cracks that catch the light as you rotate the stone
• cloudy, whitish or chalky patches inside the stone
• sand or potch lines running through the colour bar
• a dry, slightly dull surface finish

Ask how the stone has been stored, and be cautious of any opal that has been kept in a hot, dry place or moved between extreme temperatures. Once you own it, wear it, store it away from direct heat, and avoid sudden shocks. Opal contains a few per cent water and behaves accordingly.

Cut, Carat and Setting Checks

Opal is almost always cut as a cabochon, because faceting would scatter the play-of-colour rather than concentrate it. Check the dome height, the symmetry from the side, and the quality of the polish — a poor polish will dull the colour considerably. A very low dome can mean the cutter was working around a flaw, or that the stone is thinner than it looks.

Turn the stone over. A solid opal should show natural, unremarkable material at the back. A flat black or grey backing, a clean join line, or a clear dome sitting on a dark base may indicate a doublet or triplet. These are legitimate and can be excellent value, but they must be disclosed and priced accordingly.

If the stone is already set, check the edges of the setting for chips, and look closely at where the metal meets the stone for glue, uneven seating or a bezel that is hiding damage. Claw settings let you see more of the stone; bezels protect the edges but conceal flaws.

A Practical Assessment Routine

• Step one: view in daylight at arm's length, then bring it close with a loupe.
• Step two: tilt slowly through 180 degrees and note exactly where the colour appears and disappears.
• Step three: inspect the back, the edges and the surface for cracks, backing or repair.
• Step four: compare two or three stones side by side if you can — memory is a poor judge of colour.
• Step five: ask about origin, any treatment, and the return policy, and get the answers in writing.

Take your time. Opal is not a stone to be rushed into, and any reputable seller will be happy to let you study a piece properly before you decide.