The iridescent ray

Where the association came from

Four different mechanisms, one shared impression. Moonstone's blue float, called adularescence, comes from microscopic alternating layers of two feldspars that scatter short wavelengths. Labradorite's peacock flashes, labradorescence, come from lamellar structures inside the crystal reflecting at specific angles. Opal's fire comes from a regular packing of silica spheres that diffracts light like a natural grating, which was only explained in 1964 with an electron microscope. Pearl's lustre comes from stacked platelets of aragonite laid down by a mollusc.

None of these is pigment. Grind any of them to powder and the effect vanishes, because the effect lives in the structure rather than in the substance. That is a genuinely unusual property among gems and it is the physical fact underneath every story about these stones being changeable, elusive or connected to the moon.

Pearl is odder still, being the only gem in the catalogue made by a living animal and the only one that needs no cutting. It arrives finished.

What the traditions claimed

Indian tradition treats pearl, mukta, as the gem of the Moon in the navaratna, prescribed for emotional steadiness, and moonstone, chandrakanta, as literally moon-formed, said to be made of solidified moonbeams and to change with the lunar cycle. It remains a traditional gift for lovers and travellers there.

Roman writers connected moonstone to the moon on the same logic, and Pliny records the belief that its image waxed and waned.

Opal has the most instructive history in the whole library. It was prized in Rome, where Pliny describes a senator exiled rather than surrender one. Then in 1829 Walter Scott published Anne of Geierstein, in which a woman's opal loses its fire and she dies shortly afterwards. Within a year or two the European opal market had reportedly collapsed, and opal carried a reputation for bad luck for a century. Queen Victoria worked deliberately against it by giving opals as wedding presents. A single novel changed the meaning of a stone across a continent, which is worth remembering whenever someone describes a stone meaning as ancient.

Labradorite entered European knowledge only in 1770, from Labrador, and Inuit tradition holds that the northern lights were once trapped in the rock along that coast until a man struck them free with a spear.

The stones of this ray

• Moonstone (Feldspar & the feldspathoids, Mohs 6). Adularescence, that blue sheen floating under the surface, comes from layers inside the crystal scattering light. Sacred to the moon in India, where it is a traveller's gift.
• Labradorite (Feldspar & the feldspathoids, Mohs 6). Grey until the light hits it, then peacock blue and gold. Named for Labrador in 1770, and central to Inuit accounts of the aurora trapped in stone.
• Opal (Carbons, glasses & organics, Mohs 5.5). Not a crystal but packed silica spheres that split light into colour. Held unlucky in the nineteenth century because of a Walter Scott novel, which is the tradition's best cautionary tale.
• Pearl (Carbons, glasses & organics, Mohs 2.5). The only gem made by a living animal, and the only one that needs no cutting. June's birthstone, ruined by perfume, vinegar and time.

Working with the ray

The ray suits situations that have not settled. Where the red ray is for acting and the brown ray for enduring, the iridescent ray is for the period where you genuinely do not yet know, and the traditions consistently attach it to dream, intuition and the moon's cycle rather than to decision.

Moonstone is the practical member: hardness 6, affordable, and with a real and living tradition behind it in India. It cleaves easily, so it should not be worn in a ring you knock about.

Opal is the demanding one. It contains up to twenty per cent water, and it can craze, meaning develop a network of fine cracks, if it dries out or suffers a sudden temperature change. Doublets and triplets are thin slices backed and capped with other material, and they must never be soaked, because the glue fails.

Pearl is the most vulnerable object in the whole catalogue. Perfume, hairspray, vinegar, lemon juice and sweat all etch it, since it is calcium carbonate. The old rule is the right one: last thing on, first thing off.

Colour you can trust

Very little in this ray is what a beginner assumes. Most rainbow moonstone is not moonstone in the strict sense but white labradorite, which is a fair trade name but a different mineral. Much cheap moonstone is opalite, which is manufactured glass.

Opal has a large synthetic and imitation market. Created opal from laboratory processes shows a distinctive columnar or lizard-skin pattern under magnification. Doublets and triplets are legitimate and much cheaper than solid opal, and they must be sold as such. A solid black opal at a low price is not a solid black opal.

Pearls are the sharpest divide. Almost all pearls on the market are cultured, which is honest and normal; imitation pearls are glass or plastic with a coating. The tooth test still works: a real pearl, natural or cultured, feels gritty against the edge of a tooth, while an imitation feels glassy and smooth.

Labradorite is rarely faked, since good material is not expensive, but the trade name spectrolite properly refers to a particularly strong Finnish variety and is applied loosely.