Tumbled tanzanite. Image credit : D Yonick

Gemological Properties

General

Category: Sorosilicate
Variety of the mineral: Ziosite
Chemical formula: (Ca2Al3(SiO4)(Si2O7)O(OH))
Strunz classification: 09

Identification

Color: Violet to Blue, or purple.
Crystal habit: Crystals flattened in an acicular manner, may be fibrously curved
Crystal system:  Orthorhombic
Cleavage: Perfect {010} imperfect {100}
Fracture: Uneven to conchoidal
Mohs: Scale hardness 6.5
Luster: Vitreous, pearly on cleavage surfaces
Streak: White or colorless
Specific gravity: 3.10–3.38
Optical properties: biaxial positive
Refractive index: 1.69–1.70
Birefringence: 0.006–0.018
Pleochroism: Present, dichroism or trichroism depending on color.

Phenomena

Cat’s eyes are known. Many tanzanites display a color shift from blue in daylight to violet in incandescent light.

Handling

Ultrasonic: Unsafe
Extremes of Temperature: Unsafe 
The best way to care for tanzanite is to clean it with warm, soapy water.
Acids: Attacked by hydrochloric and hydrofluoric acids
Heat: Fuses under jeweler’s torch; sudden temperature changes may cause cracking.

Enhancements

Virtually all tanzanite is heat treated at low levels (approx. 500°C), which converts the brown pleochroic color to blue. This may occur naturally underground, or after mining and it is scientifically impossible to determine where or when heating occurred. As  result, heating above ground has no effect on value. The Gemological Institute of American states “Heat treatment is undetectable, but is assumed because of its prevalence”.

Geological Formation Conditions

The tanzanite deposits are hosted in metamorphic rocks, marbles and schists that belong to the Mozambique Orogenic Belt. The deposits run through the low hills of Merelani , located at the foots of Mount Kilimanjaro. Running at an angle of 41 degrees to the surface, the deposit line or horizon periodically folds over itself, creating pockets of tanzanite.

The Effect Of Lighting

How different lighting effects the color of tanzanite

Sunlight: At midday, sunlight normally has a neutral effect on the hue. Earlier and later in the day, it adds red, orange, or yellow making the stones look more purple.
Light bulbs and candlelight: Adds burgundy. Purple colors are strengthened, blue may turn violet to purple, and grayish colors may look brownish. The degree of change varies depending on the stone. Some show little change.
Fluorescent lights: Most strengthen the blue axis
Halogen spotlights: Adds sparkle and usually make stones look more purple. The color change is usually less than with light bulbs.

Further Reading

Tanzanite explained by the AGTA : general overview by the American Gem Traders Association.

“Working the Blue Seam, the Tanzanite Mines of Merelani”: an excellent article by renowned gemologist Richard Hughes, detailing the reality of tanzanite mining.