There is no universally accepted method of grading colored gemstones. Tanzanite One, a major commercial player in the tanzanite market, through its non-profit subsidiary, the Tanzanite Foundation, has introduced its own color-grading system. The new system’s color-grading scales divide tanzanite colors into a range of hues, between bluish-violet, indigo and violet-blue.
The normal primary and secondary hues in tanzanite are blue and violet. Untreated tanzanite is a trichroic gemstone, meaning that light that enters this anisotropic crystal gets refracted on different paths, with different color absorption on each of the three optical axes. As a result of this phenomenon, a multitude of colors have been observed in various specimens: shades of purple, violet, indigo, blue, cyan, green, yellow, orange, red and brown. After heating, tanzanite becomes dichroic. The dichroic colors range from violet through bluish-violet to indigo and violet-blue to blue.
Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision). The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean. Gems with eye-visible inclusions will be traded at deep discounts.