Tanzanite is a blue/violet variety of zoisite, found only in the Merelani Hills of Northern Tanzania, near Mt. Kilimanjaro, where it was discovered in 1967. Named by Tiffany and Co. for the location of the mines, tanzanite displays pleochroism, which is the ability to show different colors when the crystal is viewed from different directions.
Colors of tanzanite range from a lush, sapphire-like violet blue to more purplish hues, with both colors often visible when the stone is tilted. Originally thought of merely as an alternative to the sapphire, tanzanite is now considered one of the most revered colored gemstones. Like many gemstones, tanzanite is valued by the same classic 4 Cs as diamonds: color, clarity, cut, and carat weight.
How Tanzanite Color Affects Value
The most valuable color of tanzanite is a pure, sapphire-like blue, though a violet-blue color specific to tanzanite is highly prized as well. As a pleochroic gemstone, tanzanite will show different colors when the crystal is viewed from different directions, and it will generally be cut to maximize the best blue body color. Purple body color is also popular in tanzanite, but it is generally less valuable. Occasionally, violet blue tanzanite occurs with red flashes of pleochroic color coming from within the gem—a highly prized stone indeed.
As with most colored gemstones, highly saturated colors are more valuable than weak saturations. The bigger examples of tanzanite tend to have higher saturations, with gemstones over five carats exhibiting the deepest blues and violet blues.
Almost all naturally occurring tanzanite in its rough form is trichroic, meaning that it shows three colors—blue, violet, and brown or burgundy. In order to remove the brown or burgundy component from the natural stone, tanzanite is treated with heat to reveal the blue and violet pleochroic colors.
The first tanzanite discovered was thought to have been naturally heat treated, either by a natural heat source below the surface of the earth or from wildfire. Heat treatment of gem-quality tanzanite is nearly ubiquitous, and is widely accepted within the industry. Considered permanent and stable, the heat treatment of tanzanite does not negatively affect the value of the gemstone.
Evaluating Tanzanite Clarity
Clarity refers to the number and degree of inclusions in a gemstone—any naturally occurring flaws, cracks, or imperfections. Tanzanite frequently forms in relatively inclusion-free crystals, with cut and polished gems appearing ‘eye clean,’ meaning that any inclusions will only be detectable when viewed under magnification.
Any inclusions that can be seen with the naked eye will greatly devalue the tanzanite. Also, any inclusion-like small fractures or fissures that might compromise the stone’s durability will also lower the value dramatically. One rare inclusion can lead to chatoyancy, or a cat’s eye effect, and the resulting cat’s eye tanzanite is valued in a class by itself.
How Tanzanite Cut Affects Value
Tanzanite can be cut in a wide variety of shapes and faceting designs, but because tanzanite is pleochroic, the cut of the stone is of utmost importance to its final appearance. The relationship of the cut to the direction of the crystal structure will ultimately determine the gemstone’s face-up color—the most important factor in valuing an individual stone.
Any time a rough gemstone is cut, there is a trade-off between final carat weight and considerations of clarity and color. There is generally less waste when cutting tanzanite to get a bluish purple color than when cutting to emphasize pure blue or violet. As a result, face-up bluish purple tanzanites are more plentiful than pure blue ones. Gem cutters basically choose between larger, less desirable bluish purple finished stones and the higher per-carat price of finer blue or violet-blue.
Tanzanite Carat Weight
Carat weight is simply the unit of weight measurement for gemstones. Like most gems, the greater the carat weight of tanzanite, the more valuable the stone. With tanzanite, color is often directly related to size. Larger stones, like those above 5 carats, tend to exhibit the deepest colors, and are accordingly more highly valued. Smaller stones are usually less intense in color, and can vary from medium blue to a very light pastel, and are less expensive as a rule.
When considering carat weight of tanzanite, it is important to know that two stones of the same carat weight might appear to be very different sizes, depending on differences in each gemstone’s proportions. As a result, “face-up” measurements of the length and width are often referred to in order to compare the visual sizes of different gems.
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