Chrysoberyl (beryllium aluminum oxide)

Types/Color:

• Standard chrysoberyl – colorless, golden-yellow, greenish to brownish.

• Alexandrite – Color change from green to purple or red.

Moh’s hardness: 8.5

Refractive index: 1.744-1.755

Critical angle: 35°

Specific gravity: 3.7-3.72

Cleavage: Imperfect

Fracture: Weak, conchoidal

Dispersion: Medium (.015)

Heat sensitivity: Low, but cracks and cleavages may open up and travel with heat.

Pleochroism: Dichroic – very weak (reddish to yellowish or yellow to pale-green).

Birefringence (double refraction): Medium (.011)

Crystal structure: Orthorhombic

Treatments: May be irradiated.

Cutting: Pre-polishing with 8,000 grit helps polishing go much faster.

Polishing: Ceramic or zinc lap with 50,000 or 100,000 diamond.  Tin/lead or tin with alumina creates a good polish, but is slow.

Comments: True color changing stones exhibit a strong change in color when the light source switches from daylight or fluorescent to incandescent.  Many stones exhibit a shift in color, but true color change stones must exhibit a dramatic change, such as green to red or blue to red.  In order for a chrysoberyl to be considered an alexandrite, a strong color change must be present.  True alexandrite is extremely rare and expensive.  Beware that very good synthetic alexandrite is currently on the market.  Corundums, spinels and garnets may also display color change.