Aquamarine

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Aquamarine
Chemical composition Be3Al2(SiO3)6 Beryllium 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 it's 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 unaided eye.
Seperation techniques are given below.

Loupe

Doubling of pavillion facets may be easily detected in Zircon.

Polariscope

Aquamarine, Topaz and Zircon are anisotropic.

Glass and synthetic Spinel are isotropic and can be easily seperated using the polariscope (ADR can occur).
Topaz is biaxial and can be seperated 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 (green/blue and colorless), Zircon has weak pleochroism which may not be seen in heated blue Zircon.

"True Blue" Aquamarine may show strong dichroism.

Chelsea Colour Filter

Aquamarine: distinct green Glass (paste colored by cobalt): bright red Blue synth. Spinel: orange to red Blue Topaz: greenish Blue Zircon: greenish

Refractometer

Specific Gravity

Treatments

Routinely heat treated, which removes the green cast, leaving only the blue.

Typical inclusions

Rain inclusions