by Erum Qureshi
Aquamarines, belonging to the Beryl family, are relatively easy to identify. Various simple tests can reveal the authenticity of an aquamarine stone.
belonging to the Beryl family, are
relatively easy to identify. Various
simple tests can reveal the
authenticity of an aquamarine stone.
Aquamarine - This name is derived
from a Latin word, which means water
of the sea and so named because of
its seawater color.
Most gems have a bluish green color,
which was highly prized at one time
but has lost its charm lately. There
are also clear green
are colored by iron instead of the
chromium colored emeralds and have a
light, clear color, quite unlike the
deep green of emerald. The other is
the colorless variety, which is
named Goshenite (after Goshen County
in Hampshire County, Massachusetts).
This name, however, is fast becoming
redundant, and the name colorless
beryl or white beryl is commonly
Aquamarine belongs to the species
beryl. Beryl is a silicate mineral
having chemical formula Be3Al2
(SiO3) 6. Beryl of non-gem quality
is mined as the raw material for
beryllium, which is now increasingly
used in industry. Aquamarine
crystals are hexagonal and further,
many of them exhibit a tapering form
due to erosion.
hardness of aquamarine is 7.5
the Moh's scale
. Like all Beryls, an
aquamarine has a weak basal cleavage
and a tendency to brittleness.
Determination of Specific Gravity
The SG of aquamarine lies between
the range of 2.66 and 2.80. Some
Madagascan stones have higher
densities owing to a trace of alkali
metal in their composition. The
specific gravity or SG is measured
by immersing the stone serially in a
set of high-density liquids.
These liquids come in a set of three
or five liquids of varying
densities. They work on the simple
premise that if a stone of lesser
density is immersed it will float,
that of higher density shall sink
and one of equal density will remain
aquamarine vary from 1.572 to 1.590
for ordinary ray and 1.567 to 1.583
for the extraordinary ray.
The refractive index or RI is
measured by placing the stone face
down with a drop of contact liquid
between the polished face of the gem
and the glass of the Refractometer.
The light rays leaving one medium
(air) and entering another obliquely
(the stone) will seem to bend a
little at the place of contact. This
is called refraction of light and
the reading is taken herewith.
Aquamarines exhibit a distinct
Dichroism that is the 'twin colors',
the strength of which depends on the
depth of color of the stone - they
are deep blue and colorless, the
extraordinary ray giving the
attractive blue color. The
birefringence (difference between
the highest and lowest refractive
indices) is negative in sign and
0.005 for the lower indices and
rises to 0.007 to 0.008 for the
stones with higher indices.
When the aquamarine is viewed
through a spectroscope, the
absorption spectrum it shows
(ascribed to iron and not very
pronounced) there is a broad band in
the violet at 427 nm and a feeble
diffuse band in the blue-violet at
456nm. Further, the extraordinary
ray, which can be isolated by the
use of a Polaroid disc, shows these
bands more strongly, and in such
conditions a narrow and delicate
absorption in the middle green can
be observed at 537nm.
Aquamarine does not exhibit
luminescence. All colorless Beryls,
like the blue and sea-green stones
show a strong greenish-blue color
when viewed through the Chelsea
. In this way, aquamarines can
be picked out from a parcel of
similar looking stones.
Treatment and look-alikes
Practically all the beautiful blue
aquamarines seen in jewelry are
greenish-yellow (or even
brownish-yellow) stones, which have
been heat-treated. The blue color is
induced by heating to a temperature
between 250 and 720 degree Celsius
for a varying period. The resulting
color is permanent.
The most effective imitation of the
aquamarine is the synthetic
colored pale blue by cobalt. These
synthetics will have a totally
different refractive index (1.782)
and SG (3.63) from aquamarine.
Gemologists identify these
synthetics by placing the suspected
stone close to a strong light and
viewing it through a Chelsea Filter
held close to the eye. The synthetic
spinel shows a distinct red under
the filter, whereas aquamarine shows
a decided green.