Unheated tanzanite - some is already blue, and other pieces will need to be gently heated.
Heat Treatment Of Tanzanite
Heating Colored Gemstones
As a consumer, you will regularly encounter in the marketplace gems that have been heat-treated to change their appearance : to brighten colors or improve clarity. The heating of these stone is widely used, often using techniques little changed for hundreds of years, and an accepted industry standard.
The most commonly encountered heat-treated gems include sapphire (an estimated 90% of sapphires are heated), aquamarine (which is more green prior to heating), amethyst, ruby, and many colors of topaz and citrine.
Understanding Tanzanite Heat Treatment
Unheated Tanzanite Color
Tanzanite forms as a brownish crystal.When the crystal is gently heated, this brownish color is dissolved – and the blue and violet colors accentuated.
Many people assume that all tanzanite shows this brownish color when mined. However, in reality, a mix of brown and blue tanzanite crystals is routinely excavated. This is because some tanzanite crystals are mined closer to thermal vents in the earth, or have been warmed at some stage during their 585-million-year life by metamorphic heat, thus producing a brighter color through natural heating. By heating tanzanites that are not found in naturally heated locations, we mimic this natural process.
It should be assumed that all tanzanite has been gently heated in some way. Heating can occur naturally when the rough gem is in the earth (from volcanic activity or wildfires), or through mild heating in an oven after it has been mined. However, heat treatment is not a guarantee that tanzanite will gain a richer color, and many medium or pastel stones do not become any more deeply colored after being heated.
It is currently impossible to scientifically prove if the heating was done by man (as a treatment) vs. heating by nature. Since all tanzanite has been heated in some way (above or below ground), there is no effect on price, and all finished gems are universally assumed to have been heated.
It is important to understand that the gentle heating of tanzanite at approximately 600°C for less than an hour is quite different to the much more intense heat treatment of stones like sapphire, the majority of which are routinely heated at temperatures in excess of 1700°C (close to melting point) for days or weeks at a time, which dramatically alters the stones’ color and clarity. This is why the GIA and other major laboratories state that tanzanite has been “heated” but not “heat treated” on their certificates.