Shimmer and Shine: The Complete December Birthstone Guide Tanzanite is an awesome blue-violet variety of zoisite with the combinations of calcium and aluminum silicate. The rich and striking blue hues are a result of the trace elements of vanadium within the crystal structure, creating an alluring color theme. Talking about Tanzanite’s durability in everyday wear, it is comparatively a softer stone than other Ultimate December Birthstones like zircon or blue Topaz, with a hardness of 6.5 to 7 on the mineral scale. However, its fragility and exquisite pleochroic beauty are efficiently managed when studded in sterling silver Jewelry pieces.