Aquamarine Identification
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.
Summary: Aquamarines, 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 Beryls, which 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 used.
Species
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
The hardness of aquamarine is 7.5 on the Moh’s scale. Like all Beryls, an aquamarine has a weak basal cleavage and a tendency to brittleness.
Determination of Specific Gravity (SG)
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 suspended.
Refractive Index
The refractive indices for 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.
Pleochroism
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.
Absorption Spectra
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.
Fluorescence
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 filter. 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 spinel 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.
