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DeGwyn Gems, Private Jewelers since 1975

Tanzanite

The blue, crystalline variety of the mineral Zoisite, this gem has become one of the most popular of the blue gemstones.

DeGwyn Gems, Private Jewelers since 1975
 

History

Classification and Grading

Characteristics of Tanzanite

Evaluation

Treatments and Enhancements

Buying a Tanzanite, Things to Consider

Bibliography


History

First located in the late 1960's in Tanzania, a country in eastern Africa, this intense blue, clear, crystalline gem is a variety of the mineral Zoisite, a green, opaque material usually used in carvings and bead work.

The success of this material in jewelry and in the collecting market can be attributed to the promotion, in the 1970's, by Tiffany & Company of New York. In a full page advertisement, Tiffany & Company offered this newly discovered gemstone as, "Tanzanite."

Tanzanite has increased in popularity over the years and has become a recognized and desirable blue jewelry stone. Recently it's availability to the public was enhanced by the introduction of commercial grade material being promoted on television's home shopping networks.

Availability of finer grades of Tanzanite has always been a challenge and because the source of this finer material is limited to one area of Tanzania, rarity will always be a factor in this fine gemstone's value.

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Classification and Grading

Tanzanite can be defined as the clear, crystalline variety of the mineral Zoisite. Tanzanite's color ranges from intense blue to a violet blue. The most valuable color is similar to fine Sapphire with paler, less saturated color being less valuable.

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Characteristics of Tanzanite

  • Chemical Composition - Ca2Al3(SiO4)3(OH)
  • Crystallography - Orthorhombic Crystal System, characterized by three crystal axes at right angles with unequal length.
  • Hardness - 6 - 7 on the Moh's Hardness Table
  • Cleavage - One direction
  • Fracture - Conchoidal, splinter, brittle
  • Refractive Index: 1.691 - 1.700
  • Characteristic inclusions - Most commonly seen commercially as eye clean material
  • Fluorescence - none
  • Pleochroism: blue, purplish red, greenish yellow

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Evaluation

As with most colored gemstones, color is the most important element. Tanzanite is noted for its strong trichroism and the proper cutting orientation is essential to show the finer and desirable intense blue. It is not uncommon, however, to see stones cut to enhance weight showing the next most common coloration, purplish blue.

Intensity and evenness of color is a key evaluation in Tanzanite. Clarity is usually eye clean and although the material is notably fragile with easy cleavage, final surface finish should be crisp, clean and without abrasions.

Most Tanzanite is cut as faceted stones, with ovals and modified ovals most popular to avoid damage.

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Treatments and Enhancements

The most common treatments and enhancements of Tanzanite are:

  • Heat Treatment - rough stone is heated to very high temperature resulting in usually permanent enhancement of color

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Buying a Tanzanite, Things to Consider
 

There are three elements that should be considered when buying a Tanzanite.

Color - Look for intense pure color, neither dark or pale. Pale or uneven coloration is common in lower quality stones and should be avoided.

Clarity - Inclusions are uncommon.

Cut - Tanzanite is often cut with emphasis on creating the heaviest stone from the rough crystal instead of cutting for the beauty achieved with proper proportion and orientation.  Proper attention to orientation provides the best face up color of intense blue although the purplish blue color is more common in commercial grade stones that have been cut to retain weight.

Pricing for Tanzanite has always been variable and based on uneven supply and demand. Currently there is a large volume of small, calibrated, lighter colored material available. This material is inexpensive and is most commonly seen in commercial and home shopping network inventories. Finer quality Tanzanite continues to be a highly prized gemstone and has seen an upsurge of popularity of late with corresponding increases in prices.

Bibliography

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