gemmology.dev – Tanzanite

Introduction

Tanzanite is Tanzania's signature gemstone, a violet-blue to blue-violet variety
of zoisite (calcium aluminium silicate) found commercially only within an area
approximately 4 km by 2 km in the Merelani Hills near Mount Kilimanjaro, Arusha
Region. Discovered in 1967, reportedly by a Maasai tribesman, and named and
promoted to the trade by Tiffany & Co., it is the only gem to have been added as
a December birthstone since the original list (added 2002). Its single-source
status makes future supply finite; the deposit is expected to be exhausted within
decades, supporting long-term collector interest.

Tanzanite is strongly trichroic (showing blue, violet, and burgundy when viewed
along different crystal axes) and almost universally heat-treated at low
temperatures to remove a brown or yellow component and enhance the blue-violet
colour; detection of this treatment is generally impossible and universally assumed
in trade practice. The combination of single-locality origin, vivid colour, and
supply-constrained production makes tanzanite commercially unique among coloured
gemstones. ^[1]^[2]

Discovery & Source

The world's only commercial tanzanite deposit:

Discovery: 1967 by Maasai tribesman
Location: Merelani Hills, near Mount Kilimanjaro
Exclusivity: Only known commercial source worldwide ^[1]
Geology: Unique conditions in single deposit
Area: Approximately 4km long by 2km wide

Characteristics

Distinctive properties that define tanzanite:

Species: Zoisite (calcium aluminium silicate)
Colour: Violet-blue to blue-violet
Pleochroism: Strong; blue, violet, burgundy axes
Heat treatment: Nearly universal; improves colour ^[2]
Hardness: 6-7 (requires careful setting)
Cleavage: Perfect in one direction (care needed)

Colour Grades

Understanding tanzanite colour quality:

Exceptional

Vivid violet-blue colour
Strong saturation throughout
Medium to medium-dark tone
Commands highest prices

Fine

Strong blue with violet modifier
Attractive, well-saturated colour
Good market demand

Good/Commercial

Lighter blue-violet
Visible colour but less saturated
Accessible price points

Viewing Considerations

Direction dependence: Colour varies significantly with viewing angle
Best face-up: Cutters orient for optimal blue/violet balance
Lighting: Appears more violet under incandescent light

Heat Treatment

Standard enhancement for tanzanite:

Purpose: Removes brown/yellow component, enhances blue-violet
Prevalence: Nearly all commercial tanzanite is heated
Detection: Very difficult; generally assumed
Unheated: Rare; some collectors seek natural colour
Disclosure: Treatment generally assumed and accepted

Single Source Concern

Market Position

Tanzanite's place in the gem market:

Tiffany & Co.: Named the gem and promoted it heavily ^[1]
December birthstone: Added in 2002
Price range: Wide, from affordable to investment grade
Size availability: Fine large stones increasingly rare
Collector interest: Growing as supply concerns mount

Tanzanite