Mineral Name: Zoisite
Chemical Composition: Ca2Al3(SiO4)3(OH),
Calcium Aluminum Silicate Hydroxide.
Class: Silicates
Subclass: Sorosilicates
Group: Epidote
Crystal system: orthorhombic
Color: various shades of blue to lavender,
deeper along the crystal axis.
Refractive Index: 1.68 - 1.72
Hardness: 6.5-7
Clevage: perfect in one direction

History:
Tanzanite a member of the Epidote group is relatively new on the gemstone market.
Tanzanites were first found during the late 1960's. In the Merelani Hills of Tanzania, near Mount Kilimanjaro. Its blue-lavender color is quite popular. It has become a well known
and widely distributed gemstone. The name was changed from "blue zoisite" to tanzanite by Tiffany & Company. Some in the market have added tanzanite to the elite class of the major gemstones: Diamond, Ruby, sapphire, etc.

Other Info:
Tanzanite has only one direction of cleavage. Wich could be somewhat of a problem in most gemstones. Because a gemcutter would usually orient with the direction of strongest pleochroism to maximize the color. However there are many colors that can be seen in a tanzanite. This is because it is a trichroic gem. Very few stones are trichroic and this is what makes tanzanites very easy to identify. Trichroic refers to three layers of color. The more common layers are blues and purples but you may also see flashes of red, green, yellow, orange, or brown. The color that gemologists predominantly speak of is the "deep royal blue".

Stone Colors:
Hues that range from blue to violet blue command the highest prices. The majority of people prefer colors that look closer to a blue sapphire. Tanzanites that have a strong purple color and look like amethyst stones are much cheaper to buy. Color is the most important pricing factor for tanzanites.

Lesser stones:
may have a brownish color due to the mixing of blue, purple and green. These stones are usually heat treated to a deep blue color. Tanzanite is often confused with: Sapphire and Iolite. Sapphire is in the mineral family corundum, which is harder than tanzanite, but it is usually more heavily included. While the color range of sapphire overlaps tanzanite, it does not quite exhibit the same colors in a single stone due to the pleochroism of tanzanite. Iolite is a blue-violet gemstone variety of the mineral cordierite. Iolite has strong pleochroism and can be confused with tanzanite. However, iolite is usually less strongly colored, its pleochroic colors vary from blue-violet to yellowish gray to blue and it has less fire. Iolite's unusual color shades makes it an exotic colored gemstone whose popularity is growing day by day.

Light may affect color:
Most tanzanites have a distinct shift in color with different types of lighting. Sunlight at midday normally has a neutral effect on the hue. Earlier and later in the day sunlight adds red, orange, or yellow. Making the tanzanite look more purple. Light bulbs & candlelight Add red. Purple colors are strengthened. Blue may turn violet to purple and grayish colors may look brownish. The degrees of change varies depending on the stone. Some show little change. Flourescent lights depending on what type they are. Can strengthen the blue in tanzanite. Halogen spotlights Add sparkle and usually make stones look more purple. The color change is usually less than with light bulbs. Overcast sky can add blue and grey.

Cleaning Tips:
Soak in lukewarm soapy water using a mild liquid detergent. Rinse with water that is the same temperature and dry with a paper towel or soft cloth. You can also use window cleaner. Wipe immediately after spraying. Avoid exposing your stone to sudden changes of temperature. This includes hot tubs, very cold water, and reaching into ovens and then going straight to a cold sink. Do not wear when doing any type of physical labor.

The Future:
Recent developments at the tanzanite mines have been tragic in human terms and promise to drive the price of tanzanite much higher.