The name ‘opal’ comes from the Sanskrit word upala, literally “precious stone,” so called because these enchanting gems contain within them the colours of all other gemstones… the blue of sapphire, red of ruby, and purple of amethyst… the yellow of topaz, and the green of emerald.
According to Arabic legends, they fall to earth from flashes of lightning, and the ancient Greeks believed they guarded the wearer from sickness and granted the gift of prophecy. The Romans prized only emeralds above the opal - they carried them around as talismans of good fortune and knew them as Cupid Stones, believing their appearance similar to the clear complexion of the god of love.
More recent European lore considers the opal a symbol of hope, purity, and truth. Various accounts tell of the stone being used to improve eyesight, offer protection from wild animals, ward off evil spirits, afford the wearer invisibility, and even enhance bonds of friendship and affection. But it's not all plain sailing…
Lore: The "Opal Curse"
Why, today, is the opal considered an unlucky stone (unless you were born in October or wear them with diamonds)? The myth is a 19th century invention for which we must thank - at least in part - Sir Walter Scott. His 1829 gothic novel, Anne of Geierstein, or The Maiden of the Mist, contains a tale of one Lady Hermione, daughter of the Persian Zoroastrian traveller and magus, Danischemend, and maternal grandmother to the titular Anne. She's some sort of enchanted princess (aka witch) who possesses certain occult powers - ability to appear and disappear, pass through solid objects, etc. - possibly related to a stunning opal she wore in her hair. When, at one point in the tale, the stone is splashed with holy water:
“The opal, on which one of these drops had lighted, shot out a brilliant spark like a falling star, and became the instant afterwards lightless and colourless as a common pebble, while the beautiful Baroness sank on the floor of the chapel with a deep sigh of pain.”
I guess the connection between the opal and the misfortunate Hermione stuck in the Victorian psyche, but it probably wasn't Scott's intention. In his magnificent book, The Curious Lore of Precious Stones (1913), George F. Kunz writes that the novel:
“… contains nothing to indicate that Scott really meant to represent the opal as unlucky. [...] when a few drops of holy water were sprinkled over it, they quenched its radiance. Hermione fell into a swoon, was carried to her chamber, and the next day nothing but a small heap of ashes remained on the bed whereon she had been laid. The spell was broken and the enchantment dissolved.”
So, basically: wear your opals. They’re not cursed. Watch out for holy water. They might give you supernatural powers…
The Science Behind The Stone...
Opal is a hydrated amorphous form of silica. It contains water, anywhere from 3% to 21% by weight, and can actually dry out if left in the sun or under hot lamps. There are many varieties, black and white, common and precious, fire, jelly, boulder, wood, etc. etc. and they're usually prized for the unique iridescent 'play-of-colour' found in precious opal varieties.
The rainbow colours we see when rays of light are split into their constituent colours, then amplified (constructive interference). Move the stone through the light, and vivid flashes of colour will appear and fade - often super dramatically. Tiny silica spheres cause the light to split as it moves past them (diffraction), as the precious opal formed in seams and crevices in the base rock, these tiny, regularly-sized spheres, suspended in water, settled in layers and eventually solidified. Different sizes of sphere cause different colours to manifest - red (largest spheres) being the most rare, and thus most desirable colour to find.
White opal are best known, with colourful play-of-colour over a whiteish background, often glowing orange if the light shines through the stone. Black opal appear similar, but the colour shows over a black or deep blue background. And there are many more flavours too… boulder opal shows seams of colour flashing though the rock matrix; wood opal has formed in fossilised wood, showing the orgainic grain of the original material; fire opal is a transparent, vivid orange stone; and jelly opal is an alien-like, vitreous form of precious opal.