Appearance of Demantoid Garnet
Demantoid is the green variety of andradite garnet, a
member of the garnet group of minerals, so demantoids are always primarily
green (by definition), but the exact shade ranges from a very strong yellowish
green to nearly the color of a fine emerald. Andradite is a calcium- and
iron-rich garnet. The chemical formula is Ca3Fe2(SiO4)3 with chromium
substitution as the cause of the demantoid green color. Ferric iron is the
cause of the yellow in the stone. It has the misnomers olivine and Uralian
emerald.
Some stones have a brownish cast,
which is due to iron. Its dispersion (0.057) is unusually high, and this is
often visible as "fire" (rainbow-colored
flashes of light), although in some cases the stone's green body colour can
render this effect less noticeable. Their luster is adamantine. Demantoid also
has a high refractive index of 1.80 to 1.89.
Demantoids are generally small,
with finished stones generally under 1 carat (200 mg) and stones over 2
carats (400 mg) are rare. Stones over 3 carats (600 mg) are very rare.
Stones with
more intense green coloration are generally highly valued, but lighter stones of yellowish green
display substantially more fire. The choice of stone color or fire can
therefore be a matter of personal preference, with some preferring the more
yellowish-green stones to the green stones.
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GENERAL CHARACTERISTICS
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Category
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Mineral
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Formula (repeating unit)
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Ca3Fe2Si3O12
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IDENTIFICATION
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Color
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light to deep green
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Crystal system
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cubic
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Mohs scale hardness
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6.5–7.0
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Luster
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adamantine
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Specific gravity
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3.84
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Optical properties
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Single Refractive
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Refractive index
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1.880–1.889
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Pleochroism
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none
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Dispersion
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.057
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Common impurities
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Cr
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In approximately 2003, reports
began to circulate in the trade that some Russian demantoid garnets were being
routinely subjected to heat treatment in order to enhance their color. Such
treatment is believed to be performed at relatively low temperatures and is
thought not to be detectable by gemological testing.
Twenty-eight years after it was
discovered in 1868, gemology pioneer Max Bauer wrote that demantoid garnet
would probably never earn full-fledged gem status. Much as he admired the
stone, Bauer thought it was too small, soft and scarce to merit anything more
than curiosity. Just about the same time, the late 19th century's other great
gemology pioneer, George Kunz, was in the Ural Mountains of Russia, demantoid's
prime source, buying every piece of demantoid rough he could find. Kunz, on
leave from Tiffany's where he served as the store's chief gem buyer, was
financed by banker/tycoon J. Pierpont Morgan, an avid gem collector.
For more than a decade, George Kunz
had been a devotee of the Russian green garnet, so much so that Tiffany's made
more extensive use of the gem than any other jewelry store of the age. Indeed,
demantoid was as closely associated with Tiffany's in the late 19th century as Tsavorite, a distant relative green
grossular garnet discovered 100 years after demantoid in East Africa, is with
Tiffany's in the late 20th century. True, demantoid was a darling of upper
crust English and French jewelers. But the gem owes much of its popularity with
connoisseurs today to the Tiffany mystique-despite the fact that it has been at
least 65 years since the last significant production of Ural Mountain
demantoid. Thanks to Kunz, demantoid achieved, and still retains, an importance
far disproportionate to its availability. "Maybe one in every 10,000 pieces of Victorian jewelry used demantoid,"
says jewelry historian Joseph Gill. "Yet
you'd never think how little of it there actually was with all the fuss they
make about it today."
Telltale Inclusion: What are
Demantoid Horsetails?
Demantoid
garnet is probably the only gem whose
inclusions are considered an aesthetic property, as important as color and
brilliance. Russian demantoid often contain inclusions of chrysotile, which is a type of asbestos. These fibers radiate out
from a very small crystal of chromite. These inclusions are feathery golden
threads that tend to curve and resemble the tail of a horse, and are therefore
referred to as "horse-tail" inclusions (bundles of byssolite-a form of asbestos-that spray out in a curve from
a central chromite crystal). In gemology, the presence of such inclusions
is regarded as 'diagnostic' for natural demantoid (i.e. these inclusions are
not found in any other green gemstone). Believe it or not, the value of a stone
depends heavily on the prominence and definition of these "horse-tail" inclusions.
Some gemstones are more valuable
for their inclusions, and 'horsetails' can be regarded as desirable features in
demantoid, as they are taken as an indication of prestigious Russian origin, although some demantoids from certain
other locations (such as Italy and Iran) may also contain 'horsetails', which
are regarded as being characteristic of a serpentinite geographic origin, and,
on the other hand, not all Russian demantoids actually contain 'horsetails'.
The microstructure of some demantoids is believed to be affected by the
presence of 'horsetails' (the 'horsetail' typically originates towards the
centre of the nodule, with the fibers branching out and radiating towards the
surface), whereas horsetail-free demantoids from other sources frequently
display flat crystal faces.
Although a few yellowish-green
garnets containing horse tails have been found in the Italian Alps, this
happens so infrequently that most gem dealers still consider the horse tail to
be, in effect, a Ural Mountain birthmark. 'Technically speaking, the horse tail
isn't conclusive proof of Russian origin;' warns one gemologist. "It's just a very good indicator. To be
absolutely sure, you'd have to do chemical analyses."
Consumers may wonder why such ado
is made over finding one particular type of inclusion in the first place. Does
it really matter if a demantoid is from Russia? The answer is yes. A large part
of demantoid's mystique, historians note, is its Ural heritage. Since these
mountains also produced small amounts of alexandrite,
emerald and pink topaz, the best of which are said to be paragons of these
species, Ural mountain gems have a prestige based on locality.
This doesn't mean that a
horse-tail is all that matters when buying demantoid. But its presence
certainly helps to distinguish stones
from horse-tail-free ones found in Czechoslovakia, Arizona and, more recently,
Mexico-as well as the majority of stones from Italy. Another
difference is color. Most non-Russian
stones are so yellow (the result of coloring by iron as opposed to chromium)
that they should perhaps be called topazolite,
a greenish-yellow andradite.
Demantoid Garnet
Dispersion is Greater than Diamond
Almost all garnets are plagued by
very low dispersion (light refraction). But demantoid, a member of the
andradite family, is an exception, blessed with more of this attribute than
even diamond, a stone prized for its dispersion. No wonder, then, that the
garnet's first sellers named it demantoid (meaning
diamond-like), after the Dutch word "demant" for diamond. (In
case you're wondering why marketers used a Dutch word, keep in mind that Amsterdam
was still the world's principal diamond cutting center at the time the garnet
first came on the market.)
The new garnet's fiery brilliance
gave the stone, usually found in small sizes, a decided edge over lesser-luster
emerald and peridot, the period's leading jewelry-use green gems. Indeed, Gill
says, demantoid was often sold as "olivene" or "Uralian
emerald." That is why many pieces of Victorian gemstone jewelry made
between 1885 and 1915 feature demantoid. In fact, the stone is almost wholly identified
with the Victorian era. Luckily for demantoid, America and England had fallen
under the heavy spell of Darwin-inspired naturalism. The resulting fascination
with brute nature was manifested in jewelry design as a voguish use of bird, fish,
flower and reptile motifs. Since green symbolized nature, jewelers gravitated
toward emerald. However, motif pieces consisted largely of melee and, as said
earlier, demantoid was the green melee stone of choice among the knowledgeable.
No doubt, larger demantoids would
have figured as prominently in late 19th century jewelry, only supply prevented
it. The stone was extremely rare in sizes over 2 carats. The largest specimen
we were able to see when researching this article was a magnificent 8-carat
stone in the private collection of a New York dealer.
Demantoid Vs.
Tsavorite
The only real competition to
demantoid, color-wise, is Tsavorite,
a green grossular garnet found, so far, only in East Africa. To
traditionalists, this newer-find garnet is far inferior to Ural Mountain
demantoid. But a newer generation finds Tsavorite as praiseworthy as demantoid.
The preference battle resembles
that currently raging between fanciers of Colombian versus those of Zambian
emerald. Fine demantoid garnets, like fine Colombian emeralds, tend to have a
sweeter, livelier color, with preferred tones a bit lighter than those of their
African counterparts. But the greater gravity of Tsavorite color, like that of
African emerald, has become much less of a drawback to acceptance. Where Tsavorite
has a clear edge over demantoid is in hardness
(7 on the Mohs scale for Tsavorite, 61f2 for demantoid). That half-point difference may
not seem like much, but it translates into a decided durability edge for Tsavorite.
This helps explain the preponderance of garnet brooches and pins in estate
jewelry. Demantoid's softness made it
unwise for use in rings. The lack of large sizes also contributed to a
paucity of demantoid ring stones.
At present, demantoid is
basically a collector stone with a very small following outside this circle.
Several times in the last few years, we have seen fine demantoids that had been
bought at flea markets for a song because unsuspecting owners had no idea of
their identity or value. "If you buy
an old piece with bright green stones in it;' advises Gill, "don't dismiss them as peridots or
tourmalines. If demantoid, the value of the piece could jump considerably."
References:
Pellant, Chris. 2002.
Smithsonian Handbooks: Rocks &
Minerals (Smithsonian Handbooks).
DK ADULT; 1st edition. ISBN-10:
0789491060
Polk, Patti. 2012. Collecting
Rocks, Gems and Minerals: Identification, Values and Lapidary Uses. Krause Publication; 2 edition. ISBN-10: 1440232717
Shipley, Robert M. 1951. Dictionary of Gems and Gemology, 5th edition. Gemological Institute
of America
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