Colour Change
Colour change phenomenon in gemstones including alexandrite, colour-change garnet, sapphire, and diaspore with causes and grading.
By gemmology.dev editors
Last updated
phenomena/colour-change alexandrite chromophores lighting
Introduction
Colour-change gems display different colours under different light sources
(typically daylight vs incandescent). The most famous is alexandrite, which
appears green in daylight and red in incandescent light – often described as
"emerald by day, ruby by night."
This phenomenon results from the gem's absorption spectrum interacting
differently with various light sources.
Mechanism
How colour change occurs:
The Physical Cause
- Gem's absorption spectrum has transmission windows in both red and green
- Daylight is balanced across the spectrum → green component dominates
- Incandescent light is red-rich → red component dominates
- Eye perceives the dominant transmitted colour
Requirements
- Specific absorption spectrum with dual transmission windows
- Usually involves chromium, vanadium, or iron chromophores
- Balance between absorbed and transmitted wavelengths
- Light source spectral composition must differ significantly
Alexandrite
Alexandrite is colour-change chrysoberyl – the most famous and valuable
colour-change gem.
The Colour Change
- Daylight/fluorescent: Green to blue-green
- Incandescent: Red to purple-red
- Cause: Chromium (Cr³⁺) absorption
- Ideal: Complete change from pure green to pure red
Grading Colour Change
| Percentage | Quality Description |
|---|---|
| 100% | Complete change (pure green ↔ pure red) |
| 75-99% | Strong colour change |
| 50-74% | Moderate colour change |
| <50% | Weak colour change |
Quality Factors
- Degree of change: More complete = more valuable
- Attractiveness: Both colours should be appealing
- Saturation: Vivid colours in both lights
- Clarity: Eye-clean preferred
- Size: Large stones very rare
Alexandrite Sources
| Origin | Daylight Colour | Incandescent Colour | Notes |
|---|---|---|---|
| Russia (Urals) | Green | Red | Classic; historic; depleted |
| Brazil | Blue-green | Purple-red | Major current source |
| Sri Lanka | Olive/yellowish | Brownish-red | Often less distinct change |
| Tanzania | Green | Red | Some fine material |
| India | Variable | Variable | Emerging source |
Other Colour-Change Gems
| Gem | Daylight | Incandescent | Cause |
|---|---|---|---|
| Colour-change sapphire | Blue/violet | Purple/pink | Vanadium |
| Colour-change garnet | Blue-green | Red-purple | Vanadium + chromium |
| Colour-change spinel | Blue | Violet | Cobalt or iron |
| Colour-change diaspore | Yellow-green | Pink-red | Manganese |
| Colour-change fluorite | Blue | Purple | Rare earth elements |
Colour-Change Garnet
Some of the most dramatic colour changes occur in garnet:
Characteristics
- Usually pyrope-spessartine composition
- Can show blue-green to red-purple change
- Some match alexandrite's change quality
- Relatively rare
Sources
- Madagascar (best known)
- Tanzania
- Sri Lanka
- USA (Idaho)
Testing Light Sources
Market Considerations
Colour-change gems in the market:
- Alexandrite: Extremely valuable; strong demand
- CC sapphire: Premium over standard sapphire
- CC garnet: Significant value when strong change
- Synthetic: Synthetic colour-change gems exist (alexandrite, sapphire)
- Identification: Chemical testing may be needed for species confirmation