Tanzanite quality: colour, the D-Block system and what to look for

The tanzanite colour window

The commercially optimal colour for tanzanite is precisely characterised once you understand what the violet secondary means. Unlike other coloured stones where secondary hues are generally considered modifiers that reduce value (grey in sapphire, brown in ruby, olive in emerald), tanzanite's violet secondary is an intrinsic part of the chromophore's expression and is a quality indicator, not a quality reducer. A tanzanite without any violet, purely blue with no violet component, is not finer than one with a strong violet; it may actually be considered less characteristic (GIA; Tanzanite Foundation; Wise, 2016).

Tanzanite colour quality: what reduces value (grey, green, brown modifiers; too pale or too dark), what is acceptable commercial quality, what constitutes fine quality, and why the violet secondary is characteristic rather than detrimental. Source: Tanzanite Foundation; GIA; Wise (2016).

The grey modifier problem

The most common quality reducer in commercial tanzanite is a grey modifier. Grey-modified tanzanite looks darker and can be mistaken for "deep" colour, but the grey reduces colour saturation, making the stone look flat or inky rather than vivid. This is the mistake the buyer in the story-lede almost made: confusing grey-modified darkness with rich depth. The test is consistent across viewing angles and lighting: vivid tanzanite looks bright and saturated from all angles; grey-modified tanzanite shows a slightly dull, underwater quality (GIA; Tanzanite Foundation).

The Tanzanite Foundation D-Block grading system

The Tanzanite Foundation, working with industry partners, developed a four-tier quality grading system for tanzanite using letter grades that deliberately parallel diamond grading's use of letters, to make communication accessible to retail buyers familiar with diamond certificates. The grades run from D (finest) to A (commercial), which is counterintuitive to those accustomed to alphabetical descending value, the D designation was chosen specifically to invoke "D colour" in diamond grading parlance (Tanzanite Foundation, tanzanitefoundation.com).

The Tanzanite Foundation's four-grade quality system. D (Exceptional) is the finest tier: vivid blue-violet, no grey or green modifier, eye-clean. A (Commercial) is the volume market tier with grey modification and possible inclusions. Source: Tanzanite Foundation.

How D-Block stones differ from commercial

The D-Block designation (exceptional quality) in the Tanzanite Foundation system is not common. The vast majority of commercially produced tanzanite from Block C falls in the B and A grades, with C-grade material representing a significant but smaller portion. True D-grade tanzanite, vivid, clean, large, with no grey modifier, is the material that appears at Christie's and Sotheby's with GIA certificates and commands the prices that feature in investment discussions. It is a small fraction of total production by weight (Tanzanite Foundation; GIA; Wise, 2016).

Clarity: Type I standards apply

GIA classifies tanzanite as a Type I gemstone for clarity, meaning that inclusions are uncommon in most commercial material and eye-clean clarity is the expected standard rather than the exception. This is a significant difference from Type III emerald (where inclusions are always present) and from ruby (Type II, where significant inclusions are common). A tanzanite with obvious face-up inclusions at fine colour is a discounted stone; at the D-Block quality tier, eye-clean clarity is essentially required (GIA Gem Reference Guide, 2006, pp. 28–30; GIA Colored Stone grading).

The specific inclusions that do occur in tanzanite are primarily growth-related internal fractures, mineral crystal inclusions from the host metamorphic environment, and partially healed fractures. None of these are diagnostic for any specific quality tier, they simply reduce clarity and therefore value when they are visible to the eye (GIA; Tanzanite Foundation).

Cut and orientation: the table direction matters

Tanzanite's trichroism means that the direction in which the stone is cut directly affects its face-up colour. The cutter must orient the table facet to show the desired colour (blue or violet depending on commercial intent) along the viewing axis. Most commercial tanzanite is oriented to show blue face-up, because the pure blue-violet along the a-axis is the most commercially valued appearance. Cutters who orient the table along the c-axis would produce a brownish or burgundy stone, which is not commercially desirable after heat treatment removes that component. Some specialty cutting presents the violet axis face-up, producing stones that appear more violet than blue, a niche but legitimate cutting choice (GIA; Tanzanite Foundation; Wise, 2016).

Cut quality in tanzanite follows standard coloured stone cutting considerations: the cut should present the colour optimally (no windowing, no extinction zones), should be proportioned to give the stone depth without going too deep, and should be polished to high standard to maximise the colour's apparent richness. Tanzanite's relatively lower hardness (Mohs 6–7) means it polishes somewhat differently from corundum and requires appropriate polishing technique to avoid scratching the surface (GIA; Wise, 2016).

Size premiums in tanzanite

The size premium in fine tanzanite is meaningful but less extreme than in fine ruby or Kashmir sapphire because tanzanite production includes occasional larger crystals at the fine quality tier. Fine tanzanite at 5–10 carats with D-Block colour is rare and commands significant per-carat premiums, but the rarity is less absolute than for comparable ruby at that size. The practical size premium structure:

Approximate per-carat prices, 2024–2025. Tanzanite prices at the fine tier reflect GIA-certified D-Block or C-grade material from reputable dealers. Commercial (A-grade) material below these ranges. Sources: Tanzanite Foundation; GIA market observations; dealer benchmarks. Not price guarantees.

Lighting effects: daylight vs incandescent in tanzanite

Unlike alexandrite where the lighting change is dramatic and complete, tanzanite shows a more subtle lighting effect: the violet secondary becomes more prominent in incandescent light, shifting the face-up appearance slightly toward purple-blue. In daylight the stone appears more purely blue. Neither appearance is wrong; both are part of tanzanite's character and both should be evaluated when assessing a stone for purchase. A fine D-Block tanzanite should be vivid and attractive in both lighting conditions (GIA; Wise, 2016).

The practical buyer implication: view any tanzanite you are considering under both daylight and incandescent before purchase. A stone that appears vivid blue in the dealer's incandescent-lit display case but washes out to pale grey-blue in natural daylight has a grey modifier problem that the warm lighting conceals. The daylight test is the more revealing test for modifier detection; the incandescent test shows the violet expression at its most prominent (GIA; Tanzanite Foundation).