Spinel Identification @ Jewel Info 4 U
By: Erum Qureshi
Summary: A gemstone purported
to be a spinel undergoes stringent
tests in the lab for authenticity
and grades (i.e. quality). It has
distinctive characteristics and
traits that a gemologist can easily
identify.
image courtesy:
Orbital Joe
The mineral spinel produces a lovely
suite of gemstones which are
overshadowed by the more opulent
ruby and sapphire. After ruby,
spinel is perhaps the most beautiful
of all red gems. Its colors vary
from the lightest pink to a deep
'garnet' red. A beautiful 'flame'
orange, magnificent cobalt-blue and
a subdued blue-green are also some
color varieties of this mineral.
The derivation of the name' spinel'
is obscure; its origins lie probably
in the Latin 'spina', meaning thorn.
It may also be derived from the
Greek word, meaning spark, so
christened because of its fiery red
color.
SpeciesSpinel is a gem species by itself
and is an aluminate of magnesium.
Other members of the spinel group
are Gahnospinel, (dark green),
Gahnite (blue, dark blue), Hercynite
(dark to black), Ceylonite or
pleonaste (dark colors), Picotite
(dark green to black) and Galaxite
(dark red to black in color).
HardnessSpinel is comparatively hard,
(rating 8 on the Mohs scale of
hardness). It shows an uneven
cleavage and a conchoidal fracture
and has a brittle nature.
Determination of Specific GravityEach gemstone has its own
specific
gravity or ratio of its weight to
the same volume of water.
The specific gravity is measured by
immersing the gem (spinel in this
case) in a series of heavy liquids,
(usually a set of three liquids of
varying densities). The gem will
float in a liquid of higher density,
sink in a liquid of lower density
and remain suspended in a liquid of
the same density. Although the
density/specific gravity of spinel
varies from 3.58 to 3.98, gem grade
spinel range from 3.58 to 3.61. Pale
pink stones have much lower values.
Refractive IndexLight rays leaving one medium and
entering another obliquely seem to
bend a little at the place of
contact. This is called refraction
of light. Refractive indices for
spinel are measured by placing the
stone face down in a
Refractometer
and a reading is taken. The mineral
spinel has an RI in the range of
1.712 to 1.80. However gem quality
spinel will have a fairly consistent
RI, steady at 1.718. In red stones
rich in chromium, it may rise up to
1.74. Ceylonites have values from
1.77 to 1.80.
Absorption SpectraThe absorption spectrum of red and
pink spinel is typical of stones,
which owe their color to chromium.
In a spectroscope, which is an
instrument used to determine the
wavelength of the absorbed white
light when it is passed through the
gemstone, the spectrum is
characterized by a broad absorption
in the yellow-green, and absorption
in the violet. There are no lines in
the blue window and faint ones in
the red. The wavelength is measured
in nanometers (symbol nm) or
angstrom symbol ?. The absorption
spectra of red spinel (in
nanometers): 685, 684, 675, 665,
656, 650, 642, 632, 595-490, 465,
455. (Strong absorption lines are in
bold letters; weak ones are in
parenthesis.)
FluorescenceThe luminescence of spinel shows
considerable variation. Natural red
and pink spinel show a strong red
glow under long-wave UV light, and
weaker under short-wave
fluorescence. Dark blue spinel are
completely inert under any
radiation.
Microscopic ExaminationAs with any other gemstone, the
final call on its identification and
indeed its authenticity depends
heavily on its microscopic
examination and the ability of the
gemologist to successfully
differentiate between natural
inclusions and those commonly
associated with synthetics. Common
natural spinel inclusions include
'spangles' which are included
crystals with surrounding iridescent
stress fractures. Other common
inclusions are octahedral crystals
or cavities filled with calcite or
dolomite. Synthetic spinel have
inclusions which include gas bubbles
with torpedo shapes and long
parallel hose-like tubes. Most (but
not all) synthetic red spinel
display obvious curved growth lines
which appear as broad swathes of
color.
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