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 gray in artificial lighting. Our tanzanite buying guide will help you pick the perfect gem.

Tanzanite Buying and the Four Cs

The IGS tanzanite value listing has price guidelines for top-color tanzanite.

Color

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.

Because of its original use as an inexpensive replacement for sapphire, the most 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…