As with most fine gems, Tanzanite is valued by the four “Cs” of Color, Clarity, Cut and Carat Weight.
The deeper the saturation of color the more valued the tanzanite. This is a good general statement with some exceptions. Whether it be more blue or more violet has little effect on the value, again deeper color is more valued. When I am choosing gems to purchase I look at the gems in different lighting condition using incandescent lighting and daylight and daylight florescent lighting. As the color changes in all lights I look for gems that are attractive under all lighting conditions. Some gems have more green altering the color to a more steely blue which is of lower value. Grading systems are in use but there are no standards, so grading is up to each individual seller and therefore becomes a trust issue. Grading labs do not commit to color grading tanzanite at this time. We attempt to show color as accurately as possible through photos and videos which are enhanced to accurately depict the color. In our videos of mounted tanzanite, in order to get the depth of color of the gems accurate the color of white gold tends to show a little tone of yellow. Your white gold jewelry will be bright white.
Although the deeper colors have greater value this does not mean that some of the lighter colors like those of moderate color are not desirable. When you add a fine cut to a moderate color it can be a gem of great beauty.
Blue violet gems show more violet color and in daylight will still show a noticable component of violet color.
Violet blue color grades. Violet blue gems show more blue in incandescent light and will show little to no violet in daylight
Considered AAA Quality, Intense, Vivid and Exceptional Grades
The value of tanzanite is only significantly affected by inclusions that are eye visible. Eye clean gems would relate in G.I.A. (Gemological Institute of America) terms to those gems which are graded Si1 and higher. We have chosen to use the G.I.A. terms for clarity grading but due to the amount of time required to clarity grade tanzanite’s, the high end of our system will be gems graded VS which include stones which are flawless to gems with inclusions difficult to find under 10x magnification.
The following is our clarity grading system based on G.I.A. clarity grading for Type One colored gems.
VS Flawless to Very slightly included under 10x magnification. Desirable but only of slightly greater value than Si1
Si1 Eye clean gems with inclusions easily seen with 10x magnification. No negative effect on value.
Si2 Inclusions difficult to see with the naked eye. Slight negative effect on value of 10% to 30%.
I1 Inclusions easily seen with the naked eye. Depending on type of inclusion price is reduced by 20% to 50%
I2 Inclusion obvious to the naked eye often caused by heat treatment. Value reduced by 60% to 90%.
I3 Large or multiple inclusions that affect brilliance and are the most noticeable characteristic of the gem. Not worth owning at any price.
Cut has a significant effect on the value of tanzanite. A poor cut can affect the value of tanzanite by more than 50%. We use the following grades for cut. Each gem can be reduced in grade by some but not necessarily all of the characteristics listed for each grade, polish alone can take an otherwise very good cut gem to a good cut. Some gems can be re-cut but the average weight loss to re-cut a poorly cut gem is 30%, a 3carat gem becomes a 2 carat gem.
A top quality gem should have very good symmetry, the facets should be flat and well polished, and the facets at the culet should be cut at a steep enough angle that they reflect light and do not show a window which allows light to pass through the gem and not be reflected. The ideal main angle for cutting tanzanite is 41 degrees. An excellent cut will also have a table size typically from 50% to 65% depending on the cutting style. This grade will show brilliance across the entire gem. Prices and desirability of excellent cut due to the rarity of this quality would be 10%-15% greater than very good cuts.
Very Good Cut
Our grading of very good allows for some imperfection in polish and table sizes slightly larger than those in the excellent cut grade. Windowing due to low culet angles is still not accepted in this grading. Very good symmetry is required for this grading. A high percentage of the gem will show brilliance. This cutting quality is highly desirable but of limited availability due to restrictions on rough exports out of Tanzania. These restrictions only allow small rough to be exported mostly to Asia and most larger gems are cut in Tanzania.
Slight asymmetry, facets may not be totally flat or polish shows some lines or imperfection. Table sizes often larger than 70%. Brilliance sometimes decreased due to asymmetry and polishing issues. Slight windowing due to low culet angles. Good cutting produces many desirable gems particularly those with good clarity and fine color.
A combination of several of the following, asymmetry problems, facets not polished well and facets not flat, stones show windowing as culet angles are frequently below 35 degrees, large table sizes. Stones of this quality should be avoided or expect to pay 50% less than comparable stones with a finer cut.
Currently carat weight has less affect on tanzanite than on other gems like ruby, sapphire, emerald or diamond. If Tanzanite One Group gets more control of the market they have plans to change the pricing structure more like that of diamonds which are more affected by size and quality. We are currently in 2012 seeing the following approximate price breaks. Prices of Tanzanite change quickly and dramatically. Round, princess and trilliant cuts are often at the higher end of this listing due to limited availability. Gems showing a noticable green component will be priced significantly less.