Truly a modern gemstone, transparent zoisite of a naturally reddish brown color which can be heated to a stable blue to violet, was discovered in the shadow of Mt. Kilamanjaro in 1969. Although other varieties of opaque zoisite were well known, they made no impact on the gem market. Tanzanite’s rise to prominence among retail jewelers and the general public has been rapid and dramatic.
Although the occasional blue-violet stone is found in the rough state (Mother Nature in this case has already provided the heating); the majority of them must be heated to create this color.
In the trade, all Tanzanites are assumed to be heat treated and the color is stable. But with its hardness of 6.5, and it’s tendency to cleave, daily wear will dull the finish and its brittleness is a hazard. This lovely and expensive stone is better suited to earrings, pendants, tie pins and occasional wear rings or those with protective settings. Value
In general, stones showing more blue are valued higher than those showing more violet, and medium dark colors are the ideal. Custom cuts add value. As always, size and clarity have a strong effect on prices — large clean rough is extremely scare, so larger, fine gems are rapidly rising in price and decreasing in availability. Collector types such as greens or the ultra-rare cat’seye stones are highly sought after.