'Rainbow' Andradite Garnet

By Gary Roskin, G.G., FGA, Senior Editor -- JCK Online, 7/1/2005 2:00:00 AM

One of the more curious and rare garnets, andradite, known for its green gem dispersive variety, demantoid, also occurs in a dark root-beer-brown variety containing an iridescent phenomenon. "It looks like crystallized black opal," says Falk Burger, gem expert from New Mexico who has been mining and cutting iridescent andradite from a nearby locality. "We have about a thousand crystals," says Burger. According to his wife, Kama, Falk polishes them one step at a time, beginning with a proof window polished on one face. "The nature of the beast is such that faceted stones will be similar to diamond rose cuts, with a flat bottom and the facets on top oriented like the original crystal faces, portions of the octahedral crystal habit."

Andradite is fairly difficult to cut, notes Burger. "It cuts and polishes just like other garnets, but the color is directional and distributed within the crystal lattice much like the 'grain' in diamond." The hardness of andradite is similar to all other garnets, about 7–7.5. And while it's easy to find the iridescence, says Burger, "it's hard to maximize it."

The crystals are normally highly twined and oriented so as to make symmetrical stones wasteful to cut, notes Burger. And since the iridescent feature tends to be directional, whole-stone photography is difficult at best.

"We have about 30 crystals that we will carve, a similar number of top-grade specimens, 10 to 15 flats of specimens, and several hundred each of faceting and cabbing grade," says Burger. Typical sizes range from 1 to 14 cts. "They make great cat's-eyes," notes Burger, who also says the drusy material looks too good to be true.

There is a mélange of other minerals in the area, says Burger. And while there may appear to be plenty of rainbow andradite, the mine sight is currently under local political review.