Zoisite (Zoisite, Tanzanite)
Zoisite is a mineral once thought to be a type of tourmaline, but it was later identified as a distinct species and named after the Slovenian mineralogist Sigmund Zois. It belongs to the same epidote group as epidote; while pure specimens are colorless, impurities cause it to display a variety of colors, including pink, blue-to-violet, and green.
In particular, "Tanzanite," a trade name coined by Tiffany & Co., refers to the blue-to-violet Zoisite found in Tanzania, and it has established a firm reputation today as a high-end gemstone.
Tanzanite with a beautiful blue-violet glow
Basic Information
| English Name | Zoisite (including Tanzanite) |
|---|---|
| Japanese Name | Kairenseki (formerly referred to as Yurenseki) |
| Chemical Composition | Ca₂Al₃[OH|O|SiO₄|Si₂O₇] |
| Crystal System | Orthorhombic |
| Hardness | 6–7 |
| Specific Gravity | approx. 3.3 |
Varieties and Color Variations
- Tanzanite: Blue-to-violet Zoisite containing Vanadium. A trade name by Tiffany & Co. for stones from Tanzania.
- Thulite: Pink Zoisite colored by Manganese.
- Anyolite: A local name for opaque green Zoisite from Tanzania. Specimens containing ruby are prized as "Ruby in Zoisite" (though strictly speaking, they are sometimes evaluated as purple sapphire).
- Zoisite Cat's Eye: Specimens with well-developed tubular inclusions displaying a chatoyancy effect.
- Bicolor Zoisite: A type where two colors, such as (purple and blue) or (yellow-green and light blue), coexist in the same crystal.
Treatments and Similar Stones
Most blue Zoisite (Tanzanite) on the market has had its color stabilized by heat treatment. Ruby in Zoisite may also be heated to brighten its color, and oil or resin impregnation is sometimes performed on heavily cracked stones. Cases have also been reported where Anyolite with a pale green color is dyed with green pigment.
Blue Zoisite can resemble kyanite, while green or brown Zoisite can look like tourmaline. Opaque green varieties may be confused with "green quartz," and opaque pink ones with rhodonite or piedmontite.
Care Instructions: Zoisite has a hardness of about 6–7, but it also has cleavage, so care should be taken to avoid impact. Especially for expensive Tanzanite, its value varies significantly depending on treatments such as heating, impregnation, or dyeing, so it is safer to choose stones with explicitly disclosed treatment details.