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II. Beryl

A. Hexagonal crystal system--uniaxial (see Crystal System handout)

1. Characteristics

a. Three horizontal axes of the same length (a₁=a₂=a₃)

b. All axes intersect at 60° & 120°.

c. Fourth axis (c or optic axis) is perpendicular to the other three.

d. Six-fold symmetry around c-axis or optic axis.

2. Doubly refractive (DR)

a. Velocity of light varies with crystallographic direction, resulting in a range in refractive index.

b. All directions except optic axis (c-axis) light entering is separated into two rays.

1) Two rays vibrate in mutually perpendicular planes.

2) Two rays travel with a different velocity; therefore, has two indices of refraction or double refraction.

c. Optic axis is singly refractive; only one velocity.

B. Chemical Composition

1. Chemical group is silicate.

a. Beryllium-aluminum silicate = Be₃Al₂(Si0₃)₆

b. Pure beryl is colorless

2. Coloring agents are minute amounts of metallic oxides.

a. Emerald (green)--Cr (chromium), Va (vanadium)

b. Golden beryl--Fe³⁺ (ferric iron)

c. Aquamarine (blue)--Fe²⁺ (ferrous iron)

d. Morganite (pink)--Mn (manganese)

C. Varieties

1. Emerald

a. Medium-light to dark green, to slightly bluish-green to yellowish-green

b. May birthstone

c. Most valuable of beryl family

1) Color main value factor of 4 C's (color, clarity, cut, carat weight)

2) How mode of formation affects emerald's value

a) Forms in contact zone between igneous rocks and metamorphic rocks.

i. Igneous rocks especially granite source of beryllium-aluminum silicate composition.

ii. All beryls (except emerald) found in igneous rocks, usually pegmatites (produce large crystals, some greater than 12 feet long).

iii. Emerald's green color

· Coloring agents are Cr (chromium) and Va (vanadium) metallic oxides.

· Source of the Cr, Va are the metamorphic rocks (schists)

iv. Emerald's clarity (inclusions, fractures)

· Metamorphic rock environment causes inclusions and fractures.

· Most other beryls have cleaner clarity.

b) South America (particularly Columbia) produces finest emeralds (also Africa, Russia).

c) Iran has largest collection of emeralds in the world

d. Gem lore says that looking into an emerald "refreshes and restores vision

2. Aquamarine

a. Light to medium dark blue to blue-green.

b. March birthstone (also bloodstone).

c. Name in Latin means "sea water".

d. Very large and clean specimens found.

e. Best sources are Madagascar and Brazil (pegmatite).

3. Morganite

a. Light or medium pink, orangey-pink, purplish-pink.

b. Named after financier J.P. Morgan.

4. Goshenite or colorless beryl

5. Golden beryl

a. Light to medium dark yellow, greenish-yellow, brownish-yellow.

b. Also called Heliodor.

6. Other varieties designated by Color (i.e., light green beryl, red beryl)

7. Phenomenal varieties

a. Cat's-eye, star.