Alexandrite | Gemopedia

Alexandrite is the rare color-change variety of the mineral chrysoberyl. Its rarity is a result of its unlikely chemical makeup. Alexandrite can only form when aluminum and beryllium combine with trace elements like iron, titanium and, most importantly, chromium. On rare occasion, vanadium may also be present. The unlikelihood of the rare element chromium being in the right place to combine with aluminum and beryllium under exactly the right conditions to create alexandrite is what makes it so rare and valuable.
Colors
In Incandescent Light: Orangy, Brownish Red To Purplish Red; In Fluorescent, Daylight: Yellowish, Brownish, Grayish, Bluish Green

Alexandrite Classification

Common Name

Alexandrite

Species

Chrysoberyl

Variety

Alexandrite

Colors

In Incandescent Light: Orangy, Brownish Red To Purplish Red; In Fluorescent, Daylight: Yellowish, Brownish, Grayish, Bluish Green

Alternate Names

Gemstone Groups

Key Separations

Color change, refractive index, birefringence, pleochroism and spectrum

Comments

Alexandrite's spectrum varies by direction, showing a different spectrum in the green direction than in the purple-red pleochroic direction. Green, blue-green to pale green, yellowish, brownish, grayish in daylight or fluorescent light and violet, purplish to red, orangy or brownish in incandescent light.

Alexandrite Optical Properties

Transparency

Transparent

Refractive Index

1.746-1.755
Tolerance:(+0.004/-0.006)

Birefringence

0.008-0.01

Optic Character

Biaxial

Optic Sign

Positive

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: inert to moderate red
LWUV: inert to moderate red

CCF Reaction

red

Pleochroism

Trichroic, strong green, orange and purplish red

Dispersion

Strength: weak fire Value: 0.015

Comments

Alexandrite Chemistry & Crystallography

Chemical Name

beryllium aluminum oxide

Chemical Formula

BeAl2O4

Synthesis

Crystal System

Orthorhombic

Classification

Oxide

Nature

Natural

Crystallinity

Crystalline

Comments

Alexandrite Characteristic Physical Properties

Hardness

8.5

Streak

White

Specific Gravity

3.71-3.75 Range:+/- 0.02 Typical:3.73

Toughness

Varies

Inclusions

Alexandrite is a type II clarity stone. The stone may contain fluorite, apatite, mica platlets, rutile and multi-phase inclusions along with negative crystals, fingerprints, liquid inclusions, needles and hollow tubes. Alexandrite's might show twinning.

Luster

Bright Vitreous

Stability

Very Good

Fracture

Conchoidal

Cleavage

Good, in one direction

Comments

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