Zoisite: properties, varieties and value factors

Key properties

Chemical formula	Ca2Al3(SiO4)3(OH)
Crystal system	Orthorhombic
Mohs hardness	6-7
Toughness	Fair to poor
Refractive index	1.691-1.700
Birefringence	0.0080-0.0130
Optic character	Biaxial positive (+)
Dispersion	0.0210
Specific gravity	3.10-3.45
Cleavage	Perfect in one direction
Fracture	Conchoidal to uneven
Habit	Prismatic crystals with vertical striations, commonly flattened and heavily fractured; massive form for thulite and anyolite
Lustre	Vitreous to dull; polish: Vitreous
UV fluorescence	Generally inert
Pleochroism	Strong trichroism is the hallmark: tanzanite shows blue, violet and purplish red to greenish yellow; brown and green zoisite show green, purple and light blue or yellow-green tones
Cause of colour	Vanadium (blue-violet tanzanite), chromium (green), manganese (pink thulite); heating of brownish zoisite creates the tanzanite colour
Typical inclusions	Healed fractures, liquid and mineral inclusions, growth tubes; strong pleochroism often the identification itself; thulite is mottled massive
Stability	Stable to light and normal chemicals; attacked by hydrochloric and hydrofluoric acids
Heat sensitivity	Fuses under a jeweller's torch; sudden heat exploits the perfect cleavage
Care and cleaning	Warm soapy water safe; never ultrasonic or steam tanzanite; protect from knocks in rings
Birthstones	Tanzanite: December (added 2002); 24th wedding anniversary
Availability	Tanzanite limited but steady from one mining district; thulite and anyolite plentiful as ornamental material
Recognition	Tanzanite widely known thanks to decades of promotion; thulite familiar mainly to carvers and collectors

Varieties (2)

Tanzanite

Colour	blue to violet to bluish purple
Transparency	transparent
Clarity type	Type I; eye-clean expected after cutting
Phenomena	Rare chatoyancy
Appearance	Rich velvet blue to royal violet with a soft, almost sapphire-like depth but lower brilliance and dispersion
Trade names	tanzanite (named by Tiffany and Co after Tanzania) • block-D material (fine blue from the best mining blocks)
Misnomers	Meru sapphire is an obsolete term for tanzanite from the Merelani area near Mount Meru
Typical sizes	1 to 5 ct standard; 10 ct plus fine stones available at a price; over 50 ct rare
Cutting styles	Faceted ovals, cushions, emerald cuts; oriented to show blue across the table and violet from the side
Key separations	vs sapphire: RI 1.691-1.700 biaxial vs 1.762-1.770 uniaxial, plus tanzanite trichroism; vs iolite: RI and SG (iolite 1.54, 2.6); vs amethyst: RI 1.544; vs spinel: spinel SR; vs glass: bubbles; heating is near universal, so assume treated unless certified
Sources (localities)	Tanzania only — Merelani Hills, Arusha region, near Mount Kilimanjaro (about a 2 km strip of mining blocks)
Availability	Limited by geology to a single 4 km square mining area; steady supply of commercial grades, fine large stones increasingly scarce as deposits deepen
Market notes	One-source gem heavily promoted since 1968; heat treatment universal and accepted with disclosure; fine 5 ct plus vivid blue commands strong premiums
Care notes	Never ultrasonic or steam; warm soapy water only; perfect cleavage plus brittleness means rings need protective settings
In short	Tanzanite is the heat-treated blue-violet vanadium zoisite of Tanzania's Merelani Hills — a one-source, strongly trichroic December birthstone.

Size (ct)	USD per carat	Tier	As of
1.00–3.00	$100.00 – $500.00	commercial to fine	2026-10-26
3.00–10.00	$250.00 – $1200.00	fine	2026-10-26
10.00–30.00	$600.00 – $2500.00	fine to top	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Zoisite (other colours)

Colour	pink (thulite), green, brown, yellowish green, grey
Transparency	transparent
Clarity type	Type I for transparent faceted zoisite; massive varieties n/a
Phenomena	Rare chatoyancy
Appearance	Faceted non-tanzanite zoisite is a curiosity in pink, green or brown; massive thulite is mottled pink with white and anyolite is green rock studded with ruby crystals
Trade names	thulite (pink, also rosaline) • chrome zoisite (green) • anyolite or ruby in zoisite (green zoisite with ruby, its own reference entry) • unionite (obsolete for thulite)
Misnomers	Meru sapphire applies only to tanzanite, not these
Typical sizes	Thulite and anyolite in large pieces; transparent zoisite usually small
Cutting styles	Thulite and anyolite: cabochons, carvings, beads; transparent zoisite faceted
Key separations	thulite vs rhodonite: rhodonite harder (H 6, sg 3.4-3.7) with cleavage planes and black manganese oxide veins; vs scapolite: RI; vs rose quartz: RI and structure; anyolite is unmistakable with red ruby in green matrix; transparent green zoisite vs diopside or epidote by RI and pleochroism pattern
Sources (localities)	Tanzania (anyolite at Longido; tanzanite-area zoisite) • Norway (thulite, Sauland) • Austria • Italy • Switzerland • Mexico • USA • Australia
Availability	Thulite and anyolite plentiful; transparent non-tanzanite zoisite scarce
Market notes	Ornamental pricing by piece; thulite popular in carvings and beads; anyolite sold as ruby-in-zoisite slabs and carvings
Care notes	Warm soapy water; gentle handling for the softer massive materials
In short	Beyond tanzanite, zoisite appears as pink thulite, green chrome zoisite and the ruby-studded ornamental rock anyolite.

Size (ct)	USD per carat	Tier	As of
1.00–50.00	$1.00 – $30.00	ornamental (thulite, anyolite)	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Sources

GIA gem encyclopedia · mindat.org · Wikipedia

Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.

Key properties

Varieties (2)

Tanzanite

Zoisite (other colours)

Sources

See also