Tanzanite is the blue-to-violet variety of zoisite, coloured by vanadium, and it is a thousand times rarer than diamond by any sensible measure: it is found in one small area at the foot of Mount Kilimanjaro in Tanzania, discovered in 1967. Tiffany and Co gave it its name and launched it to fame. It is strongly pleochroic, showing blue, violet and burgundy from different directions, so cutters choose whether each stone faces up blue or violet. Nearly all tanzanite is gently heated to transform its natural brownish tones into that famous blue, a routine and openly disclosed practice. It is a modern birthstone for December.
Tanzanite is more delicate than sapphire, its obvious comparison. It has a hardness of 6 to 7 but distinct cleavage and a dislike of sharp knocks and sudden temperature changes. That makes it wonderful in pendants and earrings, and suitable for rings that are worn thoughtfully with protective settings. Never clean it ultrasonically or with steam; lukewarm, soapy water only. Its value rides on colour saturation, so a smaller vivid stone beats a large washed-out one. Synthetic look-alikes like forsterite exist, so buy from someone accountable.
Iolite and kyanite are two other blue-violet gemstones, and ruby zoisite is the very same mineral in opaque form.
