16.04: Beryl

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gemology
Gemology Project

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Beryl
Chemical composition	Be₃Al₂(SiO₃)₆Beryllium aluminum silicate
Crystal system	Hexagonal
Habit	Prismatic
Cleavage	Very difficult in one direction, rarely seen
Fracture	Conchoidal
Hardness	7.25-7.75
Optic nature	Uniaxial -
Refractive index	1.577-1.583 (+ or - 0.017)
Birefringence	0.005-0.009
Dispersion	Low, 0.014
Specific gravity	2.72 (+0.18,- 0.05)
Lustre	Vitreous to resinous
Pleochroism	Weak to moderate

Beryl is a beryllium aluminum silicate that occurs in every color of the rainbow. When green, it's usually called emerald but there exists green beryl, which is not entitled to be called emerald because its coloring agent is different. When blue, it's aquamarine and when pink, morganite. Yellow is heliodor and colorless is goshenite.
A rare raspberry red variety found in Utah is called bixbite. There is a very rare and costly variety termed Riesling beryl, that can be described as pale green colored with a warm, golden yellow flash. Two unusually dark blue types of beryl have been found as well: Maxixe beryl and True Blue Beryl. Maxixe beryl fades with exposure to light, True Blue Beryl does not.
Beryl has been used as a physician's tool and for gazing stones since ancient times. Those beliefs persist today. Beryl is metaphysically attributed with the ability to cure a number of intestinal and stomach ills, such as nausea, ulcers, and seasickness.

Characteristics

The optical and physical data of beryl can vary between varieties and localities.
Beryl belongs to the beryl group. Pezzottaite (IMA approved in 2003) is also a member of the beryl group.

Genesis

Metamorphic rocks in pegmatites

Localities

Beryl is found in many localities, among them being Brazil, India, Africa, Columbia, Australia, and Pakistan.

Habit

Prismatic with pyramidal and/or pinacoidal terminations. Often vertically striated.

Physical data

Mohs hardness: 7.25 - 7.75 (emerald is brittle).
Specific gravity: 2.7 to 2.9, depending on variety.

Optical data

Refractive index: n_ε = 1.56 n_ω = 1.59, depending on variety.
Birefringence: 0.004 to 0.009, depending on variety.
Optical sign: uniaxial negative.
Dispersion: low, 0.014.
Pleochroism: weak to moderate, strong in "True Blue Beryl" and pezzottaite.

Diaphaneity

Transparent to opaque.

Colors

All varieties of beryl are allochromatic. Main coloring agents are given below.

Green (emerald), colored by chromium 3⁺
Green (vanadium beryl), colored by vanadium 3⁺ - Also known as vanadium emerald
Green (green beryl), colored by ferric 3⁺ and ferrous 2⁺ iron
Green/Yellow (Reisling beryl), unknown coloring agent (most likely iron)
Blue (aquamarine), colored by ferrous 2⁺ Iron
Blue (Maxixe), colored by color centers
Blue (True Blue), colored by ferrous 2⁺ iron - Trade name for high FeO content aquamarine
Pink (morganite), colored by manganese 2⁺
Red (bixbite), colored by manganese 3⁺
Yellow (heliodor), colored by ferric 3⁺ Iron
Colorless (goshenite)

Enhancements

Common enhancements to beryl varieties:

Fracture filling (emerald) - oil, wax or plastic (with dye or without)
Coatings (emerald) - non-metallic paint or plastic (usually green)
Heat treatment - improves or even changes the color