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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.

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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…
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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.