Magnesio-Axinite from Merelani, Tanzania
Colored gemstone dealer Bill Vance (Vance Gems, Newark, Delaware) showed us some transparent examples of magnesio-axinite, a rare magnesium-dominant member of the axinite group of minerals. Axinite has a general formula of H(CaFe2+MnMg)3(Al2BSi4O16). Gem-quality magnesio-axinite—also known as axinite-(Mg)—is transparent to translucent, with pale blue to pale violet, light brown to light pink, or yellow to golden yellow or orange color (figure 1, left). More intensely colored bluish violet to purple magnesio-axinite tends to have a higher refractive index (1.659–1.681) than lighter-colored pink, purple, or yellow material (1.652–1.668). Magnesio-axinite has a birefringence of 0.010–0.016, strong dispersion, and a Mohs hardness of 5.5. Stones with a blue color component display pale blue to pale violet and pale gray pleochroic colors. Fluorescence is one of the material’s most striking properties—the gem shows a dull red under short-wave ultraviolet light and a vibrant “merthiolate flame color” red under long-wave UV (figure 1, right). We intend to report on the chemistry of this interesting material at a future date.
Vance said he’s attempting to name the gem “vanceite.” Besides the fluorescence, pink to blue-violet stones display a color shift: If you view it under a fluorescent light, you’re going to see a blue stone, he says, under incandescent, it’s more of a pinkish color. Yellows don’t show a color shift.
Because of the material’s color shift and strong fluorescence, Vance believes it appeals to a wide audience. According to Vance, these examples come from a very limited occurrence in the Karo pit “D” block, Merelani Hills, Tanzania, where the chemistry and formation conditions were just right, and new material is very hard to get. It comes from the same rocks as green garnet, tanzanite, and chrome tourmaline. He remarked that while cutting of magnesio-axinite is straightforward, the biggest problem is getting clean pieces to cut as almost all have inclusions. In terms of size, he said, anything over four carats is a “monster.”
Vance also showed us a heart-shaped 10.46 ct Tanzanian “Merelani mint” garnet. He noted that it displayed a very strong response to both long-wave and short-wave ultraviolet light (figure 2). Although such vanadium-bearing grossular garnet can come from Kenya or Tanzania, most of the Kenyan material displays no long-wave UV reaction. According to Vance, a strong long-wave UV reaction from a green grossular garnet is strong evidence of a Tanzanian stone from D block. Vance explained that supply of this material is now dwindling. In the beginning, prices ranged from $400–$500 per carat up to $800 per carat. Today’s prices for equivalent material are in the range of $2,500 per carat. For a particularly fine example over six carats, prices might be as high as $10,000 per carat. Demand is really strong, especially in China.