Tanzanite Buying Guide
More than just a simulant for sapphire, tanzanite’s vivid hues have stunned the gem world. Since its discovery in 1967, this gem has become very fashionable. Indeed, for blue stones, only blue sapphire is more popular. This modern December birthstone originates in Tanzania, its namesake, where the only known deposits occur. Although this rare variety of zoisite does undergo heat treatment to reach its stunning hues, the color is stable and, unlike sapphire, doesn’t grey in artificial lighting.
Tanzanite Buying and the Four Cs
The IGS tanzanite value listing has price guidelines for top color tanzanite.
Zoisite with trace vanadium content heats at a relatively low temperature to produce gem-quality tanzanite, from purple to blue. Rarely, some stones found on the surface already show these colors. These occur due to natural heating by the Sun. Fortunately, heat treatment is stable and requires no further care.
Because of its original use as an inexpensive replacement for sapphire, the top desired color is a slightly violet blue. In these cases, the slight violet hue imparts a depth and warmth to the stone.
However, some connoisseurs prefer to treat tanzanite as tanzanite rather than an off-brand sapphire. The primary difference lies in the stone’s color in natural and artificial light. Some of those seeking a replacement for sapphire may desire a gem whose hue remains a classic blue between lighting sources. On the other hand, others may appreciate a fine tanzanite’s color change ability, a shift from a rich blue in daylight to a striking purple indoors.
Ideally, tanzanite tone is 75-85%. Gems in this range have intense color with good transparency. Specimens at 85% may appear too dark, and grey hues often accompany lighter tones.
Pleochroic tanzanite exhibits blue, red to purple, and green-yellow to brown hues. This depends on the crystal axis used to view the gem. While the most valuable color is blue, the best cuts for weight retention flaunt this gem’s purple hues. Thus, lapidaries must decide whether the stone should have the most weight or the best color. Much of the tanzanite on the market is purple or violet.
Note the reflected face-up blue and purple hues of this 2.43-ct tanzanite at the beginning of the following video. These arise due to the gem’s pleochroism.
Zoisite or “Fancy Color Tanzanite?”
Rare specimens of zoisite exhibit green, yellow, or pink primary hues. However, tanzanite is the gem-quality blue to purple variety of zoisite. Other colors should be referred to as zoisite to avoid confusion. Their rarity makes them coveted collector’s pieces.
Tanzanites tend to have very good clarity. Stones with inclusions may fracture during the heating process. Thus, stone cutters cut them before undergoing treatment to ensure the removal of fractures and inclusions.
Some inclusions are needle-like and can give the stone a rare cat’s eye effect.
Due to tanzanite’s rarity, there are no standard cuts. To retain weight, lapidaries often cut the stone for the best purple color. Often, deep cuts enhance the gem’s tone. Oval and cushion cuts are the most common.
Although, generally, cutting occurs prior to treatment, some stones will benefit from a recut.
Small gems with good color are rare. Generally, stones below 2 carats won’t exhibit desirable tones, while those above 5 carats are much more likely to display top colors. Because of this, the average price per carat rises quickly for smaller stones. However, a gem with excellent color will fetch a good price at any size.
Large sizes of tanzanite exist, but gems become very rare above about 50 carats.
What is “D-Block” Tanzanite?
Four “Blocks” of tanzanite mines exist in the small area of Tanzania where the gem occurs. There is no difference in quality between the blocks. Furthermore, there is no way to tell the difference between gems mined in the D-Block from gems originating in other areas. Although some claim that the D-Block generates higher quality tanzanite, there is no basis for this.
With a hardness of 6.5, tanzanite is not the best option for daily wear. However, it is hard enough for occasional wear and for jewelry pieces like earrings, necklaces, and tie clips, which are less subject to wear. Since it’s also a brittle stone, have jewelry makers place it in a protective jewelry setting.
Tanzanite as an Investment
Any investment has risks. However, the price of tanzanite has fluctuated greatly since its discovery in 1967. The supply of the stone has wavered depending on the political situation in Tanzania, the only country where it is currently mined. Additionally, zoisite, the mineral name for tanzanite, occurs in several locations. Thus, a new discovery of gem-grade zoisite that can be heat-treated to a blue color could create further market volatility.
Beware of large tanzanite rough. Smugglers most likely removed this material from Tanzania. In 2010, a ban on the export of tanzanite rough over 5 carats became law. This allowed Tanzanians to profit from cutting and polishing rough, giving them a larger profit margin. In fact, this ban has been successful enough that Tanzania is considering expanding the ban to other gemstones, as more of their citizens become trained in lapidary techniques.
Always assume that a tanzanite is heat treated. Most material undergoes a relatively low-temperature (500-600°C) heat treatment.
Although there is no accepted method for confirming that a tanzanite specimen has been heat treated, some laboratories will certify that there is “no evidence” of heat treatment. However, brown hues along the c-axis signify that the stone may be untreated, since treatment primarily alters this hue.
Some tanzanite specimens have chemical coatings to improve color and luster. A microscope examination can reveal coatings. Generally, the coating wears away from facet edges first, which makes the difference in color between edges and faces obvious. The Gemological Institute of America (GIA) identified an iridescent coating that contained cobalt, zinc, antimony, and lead. In another case, the GIA identified a tanzanite coated with titanium.
Rumors of Dye-Treated Tanzanite
In 2012, rumors surfaced of zoisite infused with purple dye to appear like tanzanite. Supposedly, this material was cheap, plentiful, and uniform in color. These rumors have been thoroughly debunked, and gemological laboratories found that suspect specimens weren’t dyed in any way.
The observed odd coloring in surface fissures resulted from the stone’s pleochroism. The reflection of a different angle, and thus color, within the crystal was incorrectly identified as dye.
Recently, synthetic forsterite made to imitate tanzanite became available. This material is birefringent, and the back facets of a cut gem will appear doubled when viewed through the table. Additionally, its lower refractive index and green color through a Hanneman filter (as opposed to an orange-pink hue for natural tanzanite) expose its identity. (Forsterite belongs to the olivine group and, with fayalite, forms a solid-solution series that produces peridot).
Synthetic Yttrium Aluminum Garnet (YAG)
Under the brand name Tanavyte®, yttrium aluminum garnet, or YAG, with blue and purple hues, makes an excellent tanzanite simulant. Furthermore, this material has a hardness of 9, making it much more resistant to scratching than tanzanite. Tanavyte can also be identified by its refractive index of 1.8, higher than tanzanite’s, and orange fluorescence under ultraviolet light.
Coranite, a synthetic corundum, is another excellent tanzanite simulant. Again, this material is harder than tanzanite. It appears grey-green under a Hanneman filter, quite distinct from natural tanzanite.
Several kinds of man-made glass can be tanzanite simulants. Blue or purple glass, as well as blue to purple color-change glass, are inexpensive alternatives to tanzanite.