Opal Jewelry

A diffraction effect. Opal is built from microscopic silica spheres, and where those spheres are uniform in size and regularly stacked, they split white light into spectral colours.

The size of the spheres determines which colours appear - smaller spheres give blues and violets, larger ones the reds that are rarer and more valued.

Opal without that ordered internal structure shows no play of colour at all, and is called common opal.

No, and it is the only major gemstone that is not. Opal is amorphous hydrated silica - silica spheres packed together with water between them, rather than atoms in a repeating crystal lattice.

Almost everything distinctive about opal follows from that: the play of colour, the water content, the softness, and the particular care it needs.

It also means opal has no cleavage planes in the way crystalline stones such as tanzanite or topaz do.

Body colour and transparency. White opal has a pale, milky body that gives a softer, more diffuse play of colour, and it is the most familiar form.

Ethiopian opal, from a more recently worked source, is often noticeably more translucent, and its flashes can be brighter and broader as a result.

Neither is better - they look different, and it is worth seeing both before deciding which you prefer.

A warm orange to red opal valued for its own body colour rather than for play of colour.

Some fire opal shows flashes as well, but a great deal of it shows little or none, and it is bought for the saturated warm colour itself.

It is often more transparent than white opal, and it is sometimes faceted rather than cut as a cabochon because there is no internal effect that a dome is needed to display.

Because faceting reflects light off flat surfaces, and opal's interest comes from light diffracting inside the stone rather than bouncing off it.

A cabochon lets light enter, interact with the silica structure and come back as shifting colour, and the dome gathers that effect toward the viewer.

The cutting style is dictated by the physics of the material. Fire opal, which is valued for body colour rather than play of colour, is the exception and is sometimes faceted.

It is not the ideal stone for it. Opal sits at roughly 5.5 to 6.5 on the Mohs scale, among the softer stones worn as jewelry, and a hand takes more impact than anywhere else.

A bezel setting and a low profile protect it considerably, and an opal ring worn for occasions rather than constantly is entirely practical.

As a pendant or earrings, opal needs no special consideration - those pieces take a fraction of the contact a ring does.

Yes - typically several percent by weight, held within the silica structure rather than sitting in cavities.

That is why opal is sensitive to its environment in a way crystalline gemstones are not. Sudden temperature change, prolonged soaking and very dry conditions over long periods are all things to avoid.

In normal indoor wear none of this is a problem. It becomes one in an ultrasonic cleaner, a hot car or a safe-deposit box left for decades.

Crazing - a network of fine surface cracks - is the failure people worry about, and it is generally caused by thermal shock, extreme dryness or a sharp knock rather than by ordinary wear.

Avoid sudden temperature changes, never use an ultrasonic or steam cleaner, and do not leave an opal somewhere very hot or very dry for long periods.

Treated sensibly, opal has been worn for centuries. It is a stone with rules rather than a fragile curiosity.

Yellow gold is the traditional setting and picks up the warm flashes in the play of colour, which is why so much antique opal jewelry is yellow.

White metals give a cooler, cleaner background and let the blues and greens read more sharply.

The setting style matters more than the metal: a bezel protects the stone's edges and is worth choosing on any opal that will be worn regularly.

A barely damp soft cloth, and nothing more. Do not soak it, and do not use detergents or jewelry dips.

Never use an ultrasonic or steam cleaner on opal - the combination of vibration, heat and moisture is exactly what the stone cannot tolerate.

Put opal on after perfume, hairspray and lotion have dried, and store it away from harder stones and away from anywhere unusually hot or dry.