Tanzanite Quality Factors

Tanzanite is a versatile gemstone that comes in a variety of sizes and cutting styles. Although it was only discovered in the late 1960s, it has quickly established itself as one of the premier gemstones in the world.

Colour
Tanzanite is a pleochroic gem, which means it can show different colours when viewed in different crystal directions. This makes cutting a crucial element in determining the colour the consumer sees when the stone is cut and set in jewellery.

In exceptional tanzanites, the colour is an intense violetish blue with red flashes of pleochroic colour coming from within the stone. Due to the gem’s strong pleochroism, fashioned examples typically show a mix of both blue and violet colours in a wide range of tones. As with most coloured gemstones, paler colours are more available and affordable than saturated ones.

Untreated, tanzanite is typically brownish. Most blue tanzanites for sale today owe their colour to heat treatment, which is what reveals the gem’s attractive pleochroic blues and violets. The blue crystals originally discovered by Masai tribesmen were an exception because they’d probably been exposed to a natural heat source within the earth at some point.

As trade professionals assume the majority of the tanzanite on the market has been heated in some way, there’s little effect on pricing. In many cases, it’s challenging for gemmological laboratories to determine whether a tanzanite has been heated.

Clarity
Much of the tanzanite sold for jewellery has inclusions that can be seen only under magnification, so any eye-visible inclusions cause a drop in value.  Also, any inclusions that might pose durability problems, such as fractures, lower tanzanite value greatly.

Cut
Cutting plays an important role in tanzanite’s colour display. Because of its pleochroism—the ability to show different colours when viewed in different crystal directions—cutting direction determines the gem’s overall face-up colour.

Cutters consider financial realities when they decide how to cut tanzanite. As with other gems, weight lost is profit lost. Cutting a tanzanite to emphasise the violet-to-purple colour usually wastes less rough than cutting it to get a violetish blue colour. That's part of the reason why face-up violet-purple tanzanites are more plentiful than specimens with a stronger blue colour component.

But this decision must be balanced against the higher per-carat price that the finer violetish blue colour might bring. The cutter essentially makes a choice between a smaller top-colour gem and a larger violet-purple one.

Carat Weight
Tanzanite is available in a wide variety of sizes, shapes and faceting designs. The finest and deepest colours are usually seen in sizes over 5 carats. Smaller stones are often less intense in colour.