Chrysoberyl
Chrysoberyl species including alexandrite and cat's eye chrysoberyl (cymophane) with properties, colour change, chatoyancy, and identification.
Introduction
Chrysoberyl (BeAl₂O₄) is a beryllium aluminate that produces two of the most
remarkable gem phenomena: alexandrite's colour change and the finest cat's eye
effect. Despite the name, chrysoberyl is not related to beryl.
Alexandrite and fine cat's eye chrysoberyl are among the rarest and most
valuable coloured stones.
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
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 | 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:
- "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
- 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