silicates
Zoisite
Ca2Al3[Si2O7][SiO4]O(OH)
- Also known as
- tanzanitethuliteanyolite
- Crystal system
- orthorhombic
- Hardness
- 6–7 on the Mohs scale
- Lustre
- vitreous
- Colour
- Grey, green, pink, and blue as tanzanite
- Where it forms
- In metamorphic rocks. The Tanzanian occurrence formed under a very specific and rare combination of conditions.
- Chiefly from
- Merelani, TanzaniaNorwayAustriaKenya
Why it is that colour
Manganese gives the pink of thulite; chromium gives the green of the material found with ruby; vanadium gives tanzanite its blue after heating.
Safety
Safe to handle. Zoisite has perfect cleavage in one direction and is fragile for its hardness.
Where it comes from
Merelani in Tanzania is the only significant source of tanzanite. Anyolite — green zoisite with ruby crystals in it — comes from Longido in Tanzania and is widely sold as ruby-in-zoisite.
Historical use
Chiefly significant through tanzanite; see that entry.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Described in 1805 and named after Baron Sigmund Zois. No traditional attribution. Thulite has a minor Norwegian decorative history.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Zoisite is included separately from tanzanite because the relationship between them is exactly the kind this collection exists to make visible. Zoisite is the mineral. Tanzanite is a trade name for blue vanadium-bearing zoisite from one deposit, coined in 1968, and almost all of it is heated to turn it blue.
Anyolite, the green zoisite studded with opaque ruby, is a genuinely striking rock and is honestly sold under a descriptive name.
Identity checked against IMA-CNMNC master list, June 2025.