December Birthstones are Tanzanite and Blue Zircon

Tanzanite is a birthstone for December, along with zircon, turquoise, and blue topaz. Tanzanite is also the gem for a 24th anniversary.

Tanzanite, a blue variety of zoisite, was discovered not long ago in the shadows of Mount Kilimanjaro, in the Merelani Hills of Tanzania. Within a short time of tanzanite’s discovery in 1967, 90 more claims appeared in the same 20-square-mile area. No one was quite sure what the beautiful crystals were, but everyone wanted to lay claim to the profits they were certain to produce.

The new gem would eventually be known as tanzanite, and it would, at times, rival the Big 3 in popularity. The instant popularity of this transparent blue to violet to purple gem was tied to its vivid color, high clarity, and potential for large cut stones.

Tanzanite is pleochroic. When you tilt a tanzanite stone, three distinct colors can be seen in three crystal directions. Because crystals show different colors depending on viewing direction, cutters can choose bluish purple or the more favored pure blue or violet-blue hue depending on how much weight they want to retain from the rough. Often, both the violet-blue and purplish colors are readily visible in a fashioned stone when it is gently rocked and tilted.

  • MINERAL:Zoisite
  • CHEMISTRY:Ca2Al3(SiO4)3(OH)
  • COLOR:Blue to violet to bluish purple
  • REFRACTIVE INDEX:691 to 1.700
  • BIREFRINGENCE:008 to 0.013
  • MOHS HARDNESS:6 to 7


Most tanzanite is routinely heat-treated, which causes a change of color in the rough material from brown, purple or grey to a deep bluish purple to blue to purplish blue. There are no manufacturers of man-made tanzanite, though a convincing simulant exists. That means that while the simulant looks like tanzanite, it does not share the same chemical, physical or optical characteristics.

Care & Cleaning

Tanzanite jewelry is somewhat delicate and should be worn with great care. When storing tanzanite jewelry it should not be piled together, as the gems may be scratched. Tanzanites should never be put in an ultrasonic cleaner or steam-cleaned. The safest way to clean tanzanite jewelry is to use warm, sudsy water and a tightly woven microfiber cloth.

(GIA Gem Encyclopedia Tanzanite) (Jewelry Information Center)