Historically demantoid is a varietal name of andradite; the name is
in direct reference to its diamond like luster. Andradite is one of
the end member calcium garnets. At its best it is an intense blue
green devoid of inclusions. More commonly at least in the demantoid
from the Urals it is blue to yellow green with type horsetail
inclusions. In 1917, production was interrupted by the Bolshevik
Revolution only recently resuming. The major distinguishing optical
characteristics of demantoid are its dispersion, which at 0.057 is
nearly double any other garnet species. In fact I believe that it is
its dispersion that prompted the name “diamond like” rather then
luster. Generally speaking in geologic terms, demantoid garnet is
thought of as a product of skarn deposits. Skarn in simple terms being
calcium rich, contact metamorphic deposits.
In the early 1990’s there was a strike of demantoid in Mali (there is
a locality article in G&G). The market resisted the material, as it
was overly yellow and suffered from a nasty color shift when exposed
to incandescent light. About the same time that the Mali discovery
was becoming known there was a discovery of andradite on the San
Carlos reservation. First marketed by Apache Gems this material if I
recall correctly was decidedly more brown than blue green and sadly
has suffered a similar fate to the Mali demantoid.
There is a specimen of Namibian demantoid garnet on display in the
British Museum dated 1936. The Namibian area that produces this
material is on the east side of the Erongo Mountains between Omaruru
and Usakos. At its best the Namibian material begins to rival the
better grades of Russian demantoid, however such stones are
exceptionally rare in single stone sizes. Melee is much easier to
come by.
Like its Mali counterpart the Namibian material can suffer from a
nasty color shift. Study has shown that iron is the chromophore
responsible for the shift. With increasing Fe, body color shifts from
blue green towards baby sh _t brown-green while simultaneously
exhibiting a greater shift in color with exposure to incandescent
light.
As a point of nomenclature, it should be understood that there are
essentially no end member garnets. They are always a mix of multiple
species. In the case of demantoid they are generally a mix of
andradite and grossularite with Al substituting for Fe. Thus the best
demantoids will have less Fe, and more Al. producing both the better
body color and less color shift with change in light source. [a
simplification]
Two or three years ago Bill Larson at Pala International had a
selection of excellent Russian material for sale at the AGTA show in
Tucson. At about the same time the Namibian deposits were
spectacularly “rediscovered”. Approximately 3 years ago a number of
prominent German dealers came to Namibia and spent enormous sums of
money purchasing this “new” demantoid rough which they d�buted at
Tucson. In Namibia prices went as high as USD1000/gm for 10 gram
clean top color rough pieces. Unfortunately for the Germans their
timing could not have been worse as they ran face first into the
"real" demantoid. Not surprisingly the Germans augured in, leaving a
substantial smoking crater. As a result of this the demand for the
Namibian material has declined and the land rush, which it created,
has died away completely.
I have spent some time studying both the optical characteristics of
material and its economics. I wrote a white paper on the Namibian
demantoid in which I concluded that it should not be marketed as a
species equivalent to the Urals material and that its best market
would be in melee. I further suggested that the material could be
extremely useful to the estate jewelry trade as replacement stones in
Art Deco pieces particularly if custom cut for this purpose.
For every 1000 pieces of rough that I see, only 1 or 2 % is cutting
clean with the right body color. Stones of top blue green color over
1 carat are exceptionally rare. While true horsetail inclusions in
the Namibian material are exceptionally rare there have been some.
Consequently gemological identification of provenance could be very
difficult. There are substantially more 1carat+ stones in the
yellow-blue green to yellow green range. Rough or cut stones should
never be inspected for purchase in fluorescent or natural daylight. It
is much safer to purchase the material in incandescent light.
Make no mistake though; at its best in 3 to 5 carat stones the
demantoid garnets of Namibia are extraordinary gems in every way and
certainly worth 500 to 1000/ct at the wholesale level that they
currently sell for. Equivalent colored, diamond cut, calibrated
melee is available in the 75 to 120/ct range in sizes up to 5mm.
Hope this helps Bob.
@Christopher_L_Johnst
Omaruru ~ Namibia