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Garnet Varieties


#1

Hi Jess,

Malaya Garnet, appeared on the market in the late 1970’s and
come from East African deposits. Their colour are yellowish to
pinkish orange. They consist of a combination of
pyrope-spessartine, with some grossular, they may show a
spessartine spectrum, bands at 412, 424, 432, 462, 485, and
495nm.

The Umba Garnet is a colour-change pyrope-spessartite garnet and
come from the Umba River valley of East Africa. Their colour are
greenish blue by daylight and magenta by incandescent light and
usually they contain vanadium. The vanadium given a spectrum of 3
broad bands at 455, 495, and 580nm.

The Garnets provide us with one of the most complex groups of
minerals. They consist of several ismorphous series with a great
deal of intermixing. Isomorphous means : when one element in the
chimical composition of a mineral is replaced by another element
having the same valency without sensibly altering the structure
and form of the crystal. This may cause wide variations in the
physical properties. And one of the best example of this is the
Garnet group.

Species :

Almandine. Colour transparent brownish red to purplish red,
coloured by iron. RI 1.78 to 1.81, Dispersion 0.027, SG 4.10. The
diagnostic spectrum shows bands at 505, 526, 576nm and weaker
bands at 462 and 617nm. Inclusions : crystals and associated
stress cracks, acicular crystals of rutile, zircon ‘haloes’,
needles.

Andradite. Colour : the bright transparent green variety
Demantoid is coloured by chromium, it also occurs in yellow,
brownish yellow and greenish brown, the Melanite variety is
black. RI 1.89, Dispersion 0.057 (very high), SG 3.85. The
Demantoid spectrum ( due to ferric oxide) shows a cut-off at
440, 445nm and may show chromium lines at 618, 634 and may be
also at 685 and 690nm. Inclusions : the Demantoid Garnet shows
inclusions of radiating asbestos fibres known as 'horsetail’
those inclusions are diagnostic and are not seen in any other
green stones. Demantoid Garnet shows reddish (due to chromium)
through the Chelsea filter. Demantoid does not exhibit
luminescence under UV lights.

Grossular. Pure grossular garnet is colourless. The Hessonite
variety is transparent to translucent brownish yellow to brownish
red due to traces of iron and manganese. The Tsavolite variety is
transparent green coloured by vanadium and/or chromium. The
Hydrogrossular variety is a translucent jade-green coloured by
chromium, the pink is coloured by manganese, and grey-white. RI
1.74, Dispersion 0.027, SG 3.65. The dark reddish orange
Hessonite may show a spectrum at 407 and/or 430nm due to traces
of iron and manganese. The dark green Stavolite coloured by
vanadium and/or chromium may shows bands at 430 and 610nm.
Inclusions, particularly the Hessonite variety, profusion of
small crystals of apatite and/or zircon giving a treacly
appearance. The Hydrogrossular Garnet will show a
cryptocrystalline reaction under the polariscope, the pink
coloured by manganese has an RI of 1.70 to 1.72, SG 3.36 to
3.42, while the green coloured by chromium has an RI of 1.71 to
1.73 with an SG of 3.41 to 3.55, and has a strong orange-yellow
glow under UV lights.

Pyrope. Pure Pyrope, not found naturally, is colourless. Pyrope
is almost always mixed with some almandine. The colour is
transparent red to purplish red. The best Pyrope Garnet is
coloured by traces of chromium from South Africa, or iron and
manganese. RI 1.75, Dispersion 0.022, SG 3.78. The orange-red
containing chromium should show a cut-off at 440 to 445nm and a
broad band at 570nm and one pair of lines at 670 and 685nm, some
might also show a weak band at 505nm. Inclusions : occasional
needle-like crystals.

Spessartite. Often will have some isomorphous replacement by the
Almandine. Spessartine is transparent orange, reddish orange to
yellow, the colour is due to manganese. RI 1.79 to 1.81,
Dispersion 0.027, SG 3.90 to 4.20. The spectrum will show lines
in the violet and blue due to manganese, bands at 412, 424, 432,
462, 485, and 495nm, the first 3 may form a cut-off at 435nm, and
may also have bands at 505 and/or 576nm (almandine). Inclusions :
shredded-looking feathers.

Uvarovite. Rich emerald green colour, the material is
idiochromatic and colour is due to chromium. RI 1.87, Dispersion
0.030, SG 3.77. Unfortunately those crystals are too small to be
cut into gems.

Rhodolite. This Garnet is an in-between Pyrope and Almandine ,
the proportions are around 1/3 Almandine and 2/3 Pyrope. The
colour is light pink or rose and due to manganese. RI 1.75 to
1.76, SG 3.83 to 3.90. It will show a weak Almandine spectrum.
Inclusions : oriented needles and zircon ‘haloes’.

For identification : RI, spectrums, inclusions, UV lights,
polariscope (garnet belongs to the cubic system). I hope my notes
will help you, spectrums are very helpful for Garnets.

Best Regards,

Francoise.