Tanzanite rates a 6.5 on the hardness scale and has vitreous luster. It is a relatively soft and brittle stone, nowhere near as hard as a sapphire or a diamond, but given the proper care it can last a long time.
The typical color of tanzanite is blue with a violet tinge. In some lighter colored stones, the color is described as lavender. The color of the tanzanite is the most important factor in determining its price. For example, a tanzanite of the perfect blue brilliant hue in 5, 10 or 20 carats is of greater value than a paler, darker, or duller stone of twice the size and weight.
In nature, tanzanite only sometimes occurs as a blue stone, but more often is golden to brown in color. After some time, it was discovered that by heating the brown or golden variety of the stone to 600 degrees Celsius and gradually cooling it, the color could be permanently changed to the violet-blue color. Without heating, tanzanite would be unmarketable.
Tanzanite is pleochroic, meaning the purple, gray, blue and violet tones within the stone vary depending on the angle the stone is viewed from. There might also be a slight color change in incandescent light, when stones may appear to be more violet. Tanzanite’s color is caused by the amount of chromium present in the stone.
Tanzanite’s rich, royal velvety blue resembles the precious Kashmir sapphire. Only a hint of violet and a slightly less hard and lustreous appearance than sapphire, tip off experts that the stone is something else. While at one point in time, tanzanite was used as a less expensive alternative to sapphire; it is now appreciated far more for itself and far less as a sapphire substitute. Tanzanite is much softer and fragile than sapphire and can occasionally shatter. It is important therefore, if you are having your tanzanite repaired or cleaned, that you make it clear that it is a tanzanite stone so extra care may be taken.
Tanzanite is often round or oval cut. It is a stone that normally has few inclusions, but as in all gem stone families, large, clear, nearly flawless tanzanites are rare. The larger the stone, the greater the chance of flaws occurring.
Being little known and of very recent history, this stone has neither been imitated nor produced synthetically. Tanzanite’s fire factor, especially that of the splendid blue color, is comparatively high, for this reason the tanzanite alone of all the zoisite family can be classed with any other splendid gem. Tanzanite is a rare gem likely to gain in value as a collector’s item.
Tanzanite occurs in schists and gneisses as well as in metasomatic rocks together with garnet, vesuvianite and actinolite. It is also occasionally found in hydrothermal veins. Tanzanite is found in very limited quantities, almost exclusively in the northern part of Tanzania, near the Merelani hills, where the chief deposit is almost exhausted by now. However small amounts of tanzanite has also been found in Sweden, New Hampshire, Pakistan, South Africa, Kenya, Norway, Austria, and Italy.
Care must be taken when setting or handling a tanzanite. The stone is soft and brittle so it can crack if one is not careful. It is also important to avoid exposure to sudden temperature changes or heat. Tanzanite is also sensitive to pressure and has a low resistance to ultrasound so it should never be cleaned with the ultrasonic cleaners commonly used by jewelers. It can suffer irreparable damage. In general, tanzanite should be worn with care.