Gemstone encyclopedia
Every gemstone, explained
What each stone is, how it wears in jewellery, and what to ask before you choose one. Browse by colour, or go straight to a letter.
This encyclopedia is educational. Inclusion does not confirm current Ralph Jacobs stock, sourcing, availability or suitability for a particular jewellery design.
2 of 27 gemstone entries
Quick overview
Opal
Opal's flashing play-of-colour comes from tiny silica spheres. This entry covers its main types, its water content and delicacy, treatments, assembled opals, settings and care.
- Classification
- Mineraloid: amorphous silica with no crystal structure, traditionally named as a mineral
- Mohs hardness
- 5–6.5 (scratch resistance — one part of durability)
- Wear considerations
- Opal has a hardness of 5 to 6.5 on the Mohs scale and toughness rated very poor to fair, so it can be scratched or broken. It can craze if it loses moisture, and heat, very dry air, bright light and sudden temperature changes can cause cracks. A protective setting can help for people who are tough on their jewellery, and hydrophane opal should be kept out of liquid.
- Treatment
- Impregnation with oil, wax or plastic. Named here as this guide's editorial practice, not as a statement of any country's law., Sugar treatment. Named here as this guide's editorial practice, not as a statement of any country's law., Smoke treatment. Named here as this guide's editorial practice, not as a statement of any country's law., Dyeing (possible in porous hydrophane opal). Named here as this guide's editorial practice, not as a statement of any country's law.
- Care
- Clean opal with warm, soapy water, the only safe method. Keep it away from strong chemicals such as hydrofluoric acid and caustic alkaline solutions. If your opal is hydrophane, avoid immersing it…
Individual gemstones vary. Care depends on treatment, setting and condition. Encyclopedia inclusion does not confirm availability.
Individual gemstones vary · care depends on treatment, setting and condition · birthstone traditions vary · encyclopedia inclusion does not confirm availability.