Tanzanite. One source, a heated blue and why you should treat it gently.

One place on earth

IGS names the Merelani Hills in Tanzania's Lelatema Mountains as the only known source of tanzanite. Miners found it in 1967 near Mount Kilimanjaro.

Tanzanite is the blue to violet variety of the mineral zoisite. That single source limits supply. IGS notes that large clean rough is extremely scarce.

Brown rough, blue stone

Tanzanite is naturally reddish brown. IGS reports that nearly all of it is heated to roughly 500 to 600 degrees Celsius, which is 932 to 1112 degrees Fahrenheit, to produce blue to violet. It calls the treatment stable. IGS also notes that some stones receive surface coatings.

Heating is the trade standard for this stone. Ask for it in writing and ask whether the stone carries a coating. Our 22.75 ct Asscher cut is listed as heated.

Why it shows three colours

IGS describes the pleochroism as very strong. Tanzanite is trichroic, showing blue, red violet and yellow green along three crystal directions.

Turn the stone in daylight and the colour shifts with the direction you view it.

Properties and value

IGS gives a refractive index of about 1.69 to 1.70, birefringence of about 0.009 and specific gravity of about 3.35. It lists hardness as 6 to 7.

On value, IGS reports that medium dark blue violet is preferred and that more blue generally outranks more violet. Size, clarity and custom cuts also affect the price. Cat's eye tanzanite is highly prized.

IGS records that no synthetic tanzanite is known, though simulants exist. The Smithsonian holds a 122.7 ct stone.

Care

IGS notes perfect cleavage and brittleness. Together they make tanzanite risky for everyday wear. It suggests occasional pieces or protective settings.

Clean it with a soft brush, mild detergent and warm water, and keep it away from rough treatment. IGS lists tanzanite as a December birthstone, as a modern option.