16.4.1: Aquamarine
- Page ID
- 4028
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|---|---|
| Chemical composition | Be3Al2(SiO3)6Beryllium aluminum silicate |
| Crystal system | Hexagonal |
| Habit | Prismatic |
| Cleavage | Imperfect, basal, almost never seen |
| Hardness | 7.5 to 8 |
| Optic nature | Uniaxial - |
| Refractive index | 1.577-1.583 (+0.017, - 0.017) |
| Birefringence | 0.005-0.009 |
| Dispersion | Low, 0.014 |
| Specific gravity | 2.72 (+0.18,-0.5) |
| Lustre | Vitreous |
| Pleochroism | Weak to moderate |
Aquamarine is a pastel greenish-blue variety of the mineral beryl and owes its color to the presence of iron impurities. Its name is derived from the Latin words for water (aqua) and sea (marine). Crystals form in large hexagonal prisms. In ancient times, it was believed that sailors wearing aquamarine pendants would be protected from the perils of the sea. The stone symbolized happiness and eternal youth. It was viewed in Christian symbolism to bring moderation and control of passions to its owner.
Beauty is in the eye of the beholder
In the past, the natural greenish-blue color of aquamarine was considered the most desirable. Today, the usual color that we see of aquamarine is a pale pastel blue. The most sought after color is a deep blue, as typified by the "Santa Maria", "Santa Maria d'Afrique" and "Marta Rocha" aquamarine finds.
Diagnostics
Aquamarine can be confused with glass, zircon, topaz and synthetic spinel when observed with the trained unaided eye.
Separation aids of aquamarine and its common simulants are given below (all tests should be confirmed by other tests).
Color
Aquamarine is an allochromatic greenish-blue to pale blue variety of beryl, which owes its color to ferrous (2+) iron (FeO). The higher the iron content, the deeper the color. Only some localities supply the most desired medium to deep blue colors.
Amongst those are True Blue aquamarines from Canada and Santa Maria aquamarines from Brazil.
Loupe
Doubling of pavilion facets may be easily detected in zircon.
Some glass stones are casted and they will show concave facets and rounded facet edges due to shrinkage during cooling down which, however, is not diagnostic since poor (re)polishing of a genuine aquamarine may show this as well.
Polariscope
Aquamarine, topaz and zircon are anisotropic.
Glass and synthetic spinel are isotropic and can be easily separated using the polariscope (ADR can occur).
Topaz is biaxial and can be separated with the use of a conoscope.
Metamict (low-type) zircon may appear isotropic.
Dichroscope
Glass and synthetic spinel will show only one color.
Topaz is trichroic.
Aquamarine will show weak to distinct dichroism (body color and colorless). Zircon has weak pleochroism which may not be seen in heated blue zircon.
"True Blue" aquamarine may show strong dichroism.
Spectroscope
Figure \(\PageIndex{1}\): Spectrum of aquamarine
Chelsea Colour Filter
The stronger the body color, the stronger the reaction.
| Aquamarine | Distinct green |
| Glass (paste colored by cobalt) | Bright red |
| Blue synthetic spinel | Orange to red |
| Blue topaz | Greenish |
| Blue zircon | Greenish |
Refractometer
The refractive indices and optical nature of aquamarine in concert with the body color should leave little doubt about its identity. None of the common simulants will fall in its range.
Typical range for aquamarine is: nω = 1.579 - 1.593, nε = 1.564 - 1.587 with a birefringence of 0.005 to 0.006.
Specific gravity
Aquamarine will float in bromoform (a heavy liquid with SG = 2.85), while most other pale blue gemstones will sink in it.
Treatments
Routinely heat treated (at approx. 375° C), which removes the green cast, leaving only the blue.
Typical inclusions
Rain inclusions.
Feathers with 2-phase inclusions (typical for pegmatite minerals).
Long parallel channels.
Phenomena
Cat's eye.