In the October 1990 issue of the Lapidary Journal there is a
column entitled "Inclusion of the Month, Byssolite in Demantoid"
Written by Russel C. Feather ll, F.G.A., G.G., of the Smithsonian
Institution. With the kind permission of the Lapidary Journal
Managing Editor, Hazel Wheaton.
Demantoid is considered to be the rarest and most valuable of
the gem garnets. It is a yellowish green variety of andradite
garnet, a calcium iron silicate. Traces of chromium are
responsible for the green coloration and the yellow is attributed
to iron. Its specific gravity is about 3.5.
Like all garnets, demantoid is a single refractive stone, but at
about 1.89, its refractive index is quite high, even above the
limits of the standard gemological refractometer. Demantoid can
be quite brilliant, having a subadamantine to adamantine luster
and a dispersion of 0.57, higher than that of diamond’s 0.44.
Some of the fire, though, can be masked by the gem’s green body
color. Unfortunately, demantoid has a hardness of only 61/2 on
the Mohs scale and it is brittle, so it is susceptible to
scratching and chipping.
The finest demantoids have come from the Ural Mountains of
Russia, which, in 1868, is also where this variety of andradite
was first found. In circumstances not unlike those of many gem
discoveries, demantoids were discovered while prospectors were
searching in streams for gold.
In time, the source was found to be a serpentinite with narrow
veins of a fibrous form of amphibole known as byssolite, which
contained the demantoids as rounded nodules, rarely showing any
crystal faces at all. Well formed dodecahedral crystals od
demantoid, however, have been found in a similar matrix, in Val
Malenco, Italy; Dognecca, Romania; and Zermatt, Switzerland.
There is more text concerning the byssolte frequency and so
Would have included them also but my finger is tired.