[Jewelry making - Article 00720] - [Orchid] Demantoid comments

[Christopher L. Johnston]

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    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 débuted 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. 

chris AT johnston.com.na
Omaruru ~ Namibia 

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