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.
27 of 27 gemstone entries
Explore birthstones. Some months have several recognised stones and birthstone traditions vary. Association does not determine suitability, value or availability.
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Quick overview · December birthstone
Tanzanite
Tanzanite is the blue-violet variety of zoisite, mined commercially only in Tanzania's Merelani Hills. Learn how its colour is judged, why most is heated, and how to protect it.
- Classification
- Mineral variety
- Mohs hardness
- 6–7 (scratch resistance — one part of durability)
- Wear considerations
- Tanzanite is 6 to 7 on the Mohs scale, with fair to poor toughness and cleavage, so it can break when struck. It can still be worn in all types of jewellery if it is protected from rough wear and hard blows.
- Treatment
- Heat treatment (routine and stable; most blue tanzanite owes its colour to heating), Coating (rare and not permanent), Fracture filling (uncommon)
- Care
- Clean tanzanite in warm, soapy water. Ultrasonic and steam cleaners are not recommended for it. Keep it away from very high heat, sudden temperature changes, and hydrochloric and hydrofluoric acids. Protect 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.