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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

Sapphire

Sapphire is corundum in every colour except red. Learn its colour range, its heat and diffusion treatments, how durable it is for everyday rings, and what to ask before you buy.

Classification
Mineral variety
Mohs hardness
9 (scratch resistance — one part of durability)
Wear considerations
Sapphire is 9 on the Mohs scale, with excellent toughness and no cleavage, which makes it a great choice for rings worn every day. Heated stones may be slightly more brittle, so pointed corners and edges need protecting. Glass-filled stones can be damaged by mild acids, and colour from shallow diffusion can be lost through surface damage or re-cutting.
Treatment
Heating to intensify or create colour, or to reduce or strengthen silk inclusions, Lattice diffusion of certain elements, including beryllium, during heating, Fracture filling with glass (mainly high-lead glass), Oil in fissures
Care
Sapphire is 9 on the Mohs scale and resists heat, light and common chemicals in normal wear. If your stone has been heated, protect its pointed corners and edges, because heating may…
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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.