gemmology.dev – Chrysoberyl

Introduction

Chrysoberyl (BeAl₂O₄) is a beryllium aluminium oxide crystallising in the orthorhombic
system, chemically distinct from beryl despite the shared name. With hardness 8.5 Mohs,
SG 3.71–3.75, and RI 1.746–1.755 (biaxial positive, birefringence 0.008–0.010), it
is one of the harder gem species, ranking between corundum and topaz on the Mohs scale.
The species produces two phenomena-bearing varieties of extraordinary value: alexandrite
shows a chromium-driven colour change from green in daylight to red under incandescent
light ^[1], and cymophane (cat's eye chrysoberyl) shows the
sharpest chatoyancy of any gem material due to dense parallel silk inclusions. Classic
Russian Ural alexandrite, essentially unavailable today, defined the benchmark colour
change (emerald green to raspberry red) and stones with documented Ural origin command
extraordinary auction premiums. ^[2] Fine cat's eye chrysoberyl
with honey-yellow body colour and the "milk and honey" side-lit effect is similarly prized;
only chrysoberyl may legally be called simply "cat's eye" without further qualification. ^[3]

Mineralogy

Crystal System and Structure

Crystal system: Orthorhombic
Chemical formula: BeAl₂O₄
Habit: Tabular crystals, often cyclic twins
Cleavage: Good in one direction
Twinning: Trillings (three-part twins) common

Physical Properties

Property	Value
Hardness	8.5 Mohs
Specific gravity	3.71–3.75
Refractive index	1.746–1.755
Birefringence	0.008–0.010
Optic character	Biaxial positive
Pleochroism	Strong (trichroic) in alexandrite
Lustre	Vitreous

Ordinary Chrysoberyl

Non-phenomenal chrysoberyl occurs in various colours:

Yellow to golden: Most common gem chrysoberyl
Green: Without colour change
Brown: Less desirable
Colourless: Rare

Properties: Hard, durable, good brilliance. Less valuable than
alexandrite or cat's eye but an excellent gem material.

Alexandrite

Alexandrite is colour-change chrysoberyl, one of the most remarkable
and valuable gem phenomena.

The Colour Change

Daylight/fluorescent: Green to blue-green
Incandescent: Red to purple-red
Cause: Chromium (Cr³⁺) absorption spectrum ^[1]

The alexandrite effect occurs because chromium creates a
transmission window between red and green, and light sources
with different spectral compositions reveal different colours.

Grading Colour Change

Colour change is assessed as percentage shift:

100%: Complete change (green ↔ red, no overlap)
75–99%: Strong colour change
50–74%: Moderate colour change
<50%: Weak colour change

Both colours should be attractive; muddy colours reduce value.

Major Sources

Origin	Characteristics	Market Position
Russia (Urals)	Classic green/red change; historic ^[2]	Highest premium (depleted)
Brazil	Good change; more bluish-green	Major current source
Sri Lanka	Often yellowish; variable change	Historic source
Tanzania	Some fine material	Limited production
India	Variable quality	Emerging source

Trichroism

Alexandrite shows strong trichroism:

Green / orange / purple-red in three directions
Visible with dichroscope
Affects face-up colour depending on cut orientation
Diagnostic feature for natural alexandrite

Russian Alexandrite

Cat's Eye Chrysoberyl

Chrysoberyl cat's eye (cymophane) shows the finest chatoyancy of any gem.

The Chatoyant Band

Cause: Parallel silk (needle inclusions)
Effect: Single sharp band of light
Quality: Sharpest, most defined of any cat's eye
Cut: Must be en cabochon perpendicular to needles

The Milk and Honey Effect

When lit from one side, fine cat's eye shows:

"Milk" side: Lighter, more opaque
"Honey" side: Darker, more transparent
Indicates dense, even silk distribution
Sign of highest quality

Quality Factors

Factor	Premium Quality
Eye sharpness	Crisp, knife-edge line
Eye movement	Smooth, fluid motion
Body colour	Honey yellow to greenish
Milk and honey	Strong contrast when side-lit
Transparency	Translucent (not too opaque)

Nomenclature

Only chrysoberyl may be called simply "cat's eye"; all other
chatoyant gems require qualification: ^[3]

"Cat's eye" = chrysoberyl cat's eye
"Quartz cat's eye" = chatoyant quartz
"Tourmaline cat's eye" = chatoyant tourmaline

Cat's Eye Value

Synthetic and Simulants

Be aware of synthetics and simulants:

Synthetic Alexandrite

Czochralski-pulled alexandrite exists ^[3]
Shows colour change but often different colours
May show curved striae or gas bubbles
Flux-grown synthetic also produced

Colour-Change Simulants

Synthetic corundum with colour change
Synthetic spinel with colour change
Both lack chrysoberyl's RI and SG
Chemical composition differs

Identification Summary

Key features for chrysoberyl identification:

RI: 1.746–1.755 (distinctive range)
SG: 3.73 (characteristic)
Birefringence: 0.008–0.010
Hardness: 8.5 (very hard)
Trichroism: Strong in alexandrite
Colour change: Diagnostic for alexandrite
Chatoyancy: Superior in cat's eye

Chrysoberyl