Opal
Opal breaks the usual rule that gemstones are crystals. It’s a mineraloid — amorphous hydrated silica (SiO₂·nH₂O) with no ordered crystal lattice at all — which sets it apart from the quartz family it’s chemically related to. What makes precious opal valuable isn’t its chemistry so much as its internal architecture: a regular, closely-packed array of microscopic silica spheres that diffracts white light into flashes of spectral color as the stone is turned. This “play-of-color” only appears when the spheres are uniform and tightly ordered; common opal, which lacks that structure, is opaque or translucent with no color play and is a decorative material rather than a gemstone in the trade sense.
Australia has long supplied the majority of the world’s precious opal, particularly the vivid black and white opal from South Australia and New South Wales, while Ethiopia has become a major modern source, alongside Mexico, Brazil, and Peru.
The trait that most separates opal from other gems is water. Opal typically holds several percent water within its structure, and that water is the reason opal is genuinely more fragile in daily life than its Mohs hardness of 5.5 to 6.5 would suggest. Rapid drying — from sustained heat, direct sun, or very low humidity — can cause the stone to lose water unevenly, opening up a fine network of internal cracks called crazing that dulls the stone’s brilliance and, in bad cases, causes visible fracturing. Ultrasonic and steam cleaners are a real risk for the same reason. The honest care advice is unglamorous: keep opal away from prolonged heat, store it with a bit of humidity rather than in a bone-dry safe, and clean it only with a soft, slightly damp cloth.
Opal’s birthstone standing is unusually consistent. It’s the October stone on every monthly list tracked here — historical, US 1912, UK 2013, and current US — sometimes paired with a second stone (aquamarine historically, tourmaline in the two US lists) but always present. That consistency doesn’t carry over to the zodiac tables, though: opal is Libra’s stone only in the modern jewelry-trade zodiac list, which maps the current monthly calendar onto sign dates. The older tropical zodiac instead gives Libra chrysolite, following the historical September entry rather than October — one more case where the two zodiac systems, which name a different stone for all twelve signs and agree on none, tell genuinely different stories about the same date range.
Gemmological properties
| Mohs hardness | 5.5–6.5 |
|---|---|
| Crystal system | Not recorded |
| Chemical formula | SiO2·nH2O |
| Refractive index | 1.44–1.46 |
| Color family | Multicolour, White, Colourless |
| Variety of | — |
| Also known as | — |
| Origins | Australia, Ethiopia, Mexico, Brazil, Peru |
| Organic material | No |
Opal is a mineraloid — hydrated amorphous silica with no true crystal structure — rather than a crystalline mineral; its play-of-color comes from regular arrays of silica microspheres diffracting light.
Where Opal appears as a birthstone
By month
| Month | System | Confidence |
|---|---|---|
| October | Historical (15th–20th century) | high |
| October | UK 2013 | high |
| October | US 1912 | high |
| October | US current | high |
By zodiac sign
| Sign | System | Confidence |
|---|---|---|
| Libra | Modern jewellery-trade zodiac | high |
Frequently asked questions
- Why does opal need special care?
- Opal is a mineraloid — hydrated silica with no crystalline structure — and typically contains several percent water by weight. Extreme dryness, sudden temperature changes, or exposure to heat can cause that water to escape unevenly, leading to crazing: a network of fine internal cracks that dulls or fractures the stone. Ultrasonic cleaners and prolonged direct sun or heat are the main things to avoid.
- What causes opal's play-of-color?
- A regular, three-dimensional array of tiny silica microspheres inside the stone, packed closely enough to diffract light into spectral colors as the stone moves. Common opal lacks this ordered structure and shows no play-of-color at all — only precious opal has it.
- Is opal really October's birthstone across every list?
- Yes, uniquely among the months tracked on this site — opal appears on all four monthly systems (historical, US 1912, UK 2013, current US) and on the modern jewelry-trade zodiac list for Libra, though the older tropical zodiac assigns Libra a different stone entirely (chrysolite).