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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

Onyx

Onyx is banded chalcedony, a fine-grained quartz. Most stones sold as black onyx are dyed. It rates 6.5–7 on the Mohs scale and scratches fairly easily when polished.

Classification
Mineral variety
Mohs hardness
6.5–7 (scratch resistance — one part of durability)
Wear considerations
Hardness of 7 on the Mohs scale (one gem database gives 6.5–7), with fairly good toughness. Polished surfaces scratch fairly easily and the scratches show. Black onyx is sensitive to heat, so jewellers should remove it before a ring is resized or its prongs are re-tipped.
Treatment
Sugar-and-acid carbon treatment that turns the stone black (traditional method; the colour is permanent). Trade guidance says treated stones should technically be described as 'dyed' or 'enhanced' black chalcedony., Dyeing with organic dyes (the colour may fade in direct sunlight). Trade guidance says treated stones should technically be described as 'dyed' or 'enhanced' black chalcedony.
Care
Clean onyx gently with warm water, a little mild soap and a soft brush. Rinse it, then let it dry fully before you wear or store it. Polished onyx scratches fairly easily,…
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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.