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Gemmological Properties

General

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

Identification

Color: Purple to blue
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. The Gemmological Institute of American states “Heat treatment is undetectable, but is assumed because of its prevalence”.  

Geological Formation Conditions   

The conditions involved in tanzanite’s formation 585 million years ago saw the random incorporation of vanadium during an event so unique that experts believe the chance of tanzanite occurring elsewhere is one in a million. 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 that rise from the hot plains at the foot 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 Ligthing

How different lighting effects the colour of Tanzanite 

Sunlight: At midday, it 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: Add sparkle and usually make stones look more purple. The colour change is usually less than with light bulbs.

Useful Information

Tanzanite Geology

Tanzanite Mining