Alexandrite Effect – Physical Mechanism
Deep dive into the alexandrite effect – Cr³⁺ in trigonal crystal field, the dual transmission window, photopic peak balance, named species including garnets, sapphire, and synthetic spinel, and how to test colour change under standardised illuminants.
Definition
The alexandrite effect is the reversible, repeatable shift in apparent body colour of a
gemstone when the illuminant changes from daylight (approximately 6500 K, blue-rich) to
incandescent light (approximately 2700–3200 K, red-rich).
It is caused by a dual transmission window in the gem's absorption spectrum: the stone
transmits both red (650–700 nm) and blue-green (450–510 nm) wavelengths simultaneously.
Which colour the eye perceives as dominant depends on the spectral power distribution of
the illuminant in those two regions.
This page provides the physical mechanism underpinning colour change as described in the
overview page. For grading, quality factors, and broader colour-change gem survey, see
the companion page on Colour Change.
Mechanism
The crystal field physics of Cr³⁺ in chrysoberyl:
Chromophore and Crystal Field
In alexandrite (the colour-change variety of chrysoberyl, BeAl₂O₄), the colour-active
ion is Cr³⁺ substituting for Al³⁺ in a distorted octahedral site. The octahedral
crystal field splits the Cr³⁺ d-orbital energy levels, creating two broad absorption
bands:
- One centred around 550–580 nm (yellow-green)
- One in the UV/violet (~400 nm)
These two bands leave two transmission windows:
- Red window (~650–700 nm)
- Blue-green window (~450–510 nm)
The stone transmits both simultaneously – this is the structural prerequisite for the
alexandrite effect.
The Photopic Peak and Illuminant Dependence
Human daylight (photopic) vision is most sensitive near 555 nm. The Cr³⁺ absorption
at ~560 nm sits exactly at the photopic peak, suppressing the eye's most sensitive
wavelength range. What remains to dominate the perceived colour depends entirely on
illuminant composition:
- Daylight (D65, ~6500 K) – strong blue-green component in the illuminant
preferentially excites the blue-green transmission window → stone appears green to teal - Incandescent (~2700 K, tungsten) – weak blue, strong red in the illuminant
preferentially excites the red transmission window → stone appears red to purplish-red
Qiu & Guo (2021) confirmed this mechanism in pyrope-spessartine colour-change garnets:
"As they exhibit the same capacity to transmit light, the colour of the gem is
determined by the external light source."
Crystal Field Context
Why does Cr³⁺ produce red in ruby, green in emerald, and teal-green/red in alexandrite?
The octahedral crystal field strength (Δo) differs by host mineral:
- Corundum (ruby): Strong octahedral field → Cr³⁺ absorbs at shorter wavelengths
→ absorption band shifts toward blue-green → red light passes → red colour - Beryl (emerald): Weaker field → absorption band shifts toward longer wavelengths
→ red light partially absorbed → green light passes - Chrysoberyl (alexandrite): Intermediate field → two absorption bands straddle
the visible spectrum, leaving both red and blue-green to pass → alexandrite effect
Named Species
| Species / Variety | Active Ion(s) | Daylight Colour | Incandescent Colour | Quality / Notes |
|---|---|---|---|---|
| Alexandrite (chrysoberyl var.) | Cr³⁺ | Green to teal | Red to purplish-red | Strongest and most complete change known; benchmark for all colour-change gems. Schmetzer & Malsy (2011) [VERIFIED] |
| Colour-change garnet (pyrope-spessartine) | Cr³⁺ + V³⁺ | Blue-green to teal | Purple to red | Very strong; best specimens rival alexandrite in completeness. Qiu & Guo (2021) [VERIFIED] |
| Colour-change sapphire (corundum) | V³⁺ (± Cr³⁺) | Blue to violet | Purple to red | Variable; often greyish intermediate colour; locality affects quality |
| Colour-change diaspore ('zultanite', 'csarite') | V³⁺ + Cr³⁺ (proposed) | Kiwi green | Pinkish champagne | Subtle but characteristic change; spectroscopic assignment [PARTIALLY_SUPPORTED] – no dedicated DOI-verified primary paper retrieved |
| Colour-change synthetic spinel (Co-doped) | Co²⁺ | Blue-green | Red-pink | Common in older synthetic 'alexandrite' simulants; chalky blue-white SW fluorescence distinguishes from alexandrite |
| Colour-change fluorite (some) | [CITATION NEEDED] | Various | Various | Weak; mechanism not confirmed; not diagnostically significant |
How to Test the Alexandrite Effect
Standardised testing conditions for colour-change assessment:
Light Source Requirements
- Daylight equivalent: D65 fluorescent lamp or north-facing window light (overcast sky).
Avoid mixed LED lighting – LED spectra vary widely and may not reproduce the effect
consistently. - Incandescent: Standard tungsten bulb (~2700 K). Not LED or halogen at high colour
temperature; these do not have sufficient red component to show the full change.
Observe the stone under each source in turn and note the dominant hue in each condition.
Grading the Change
- Complete change (100%): Pure green/teal in daylight; pure red in incandescent; no
intermediate or residual colour of the other type - Strong (75–99%): Clearly different hues with minor residual component
- Moderate (50–74%): Distinct but less dramatic shift
- Weak (<50%): Slight shift only; marginally perceptible
Laboratory reports use CIE colorimetry under D65 vs Standard Illuminant A to quantify
change objectively.
Species Identification After Colour Change Confirmation
Confirming the alexandrite effect does not identify the species. Use RI, SG, and
spectroscopic data to determine the mineral:
- Green → strong red; RI 1.746–1.755; SG ~3.73 → alexandrite (chrysoberyl)
- Blue-green → strong red; RI 1.740–1.760; SG ~3.8–4.0 → colour-change garnet
- Blue-grey → weak purple; RI 1.760–1.770; SG ~4.0 → colour-change sapphire
- Green → pinkish champagne; RI 1.700–1.750; SG ~3.3–3.4 → diaspore
Synthetic Alexandrite Diagnostics
Separating natural alexandrite from synthetic varieties:
Growth Features
Natural alexandrite shows planar growth zones (fingerprints, two-phase fluid inclusions,
chrysotile fibres) typical of metamorphic or pegmatitic origin. Schmetzer, Bernhardt &
Hainschwang (2013) documented titanium-bearing synthetic alexandrite grown by flux and
hydrothermal methods, noting curved growth planes and specific UV-Vis absorption
differences (Ti⁴⁺ bands) absent in natural material.
Fluorescence Contrast
Natural alexandrite: moderate red LWUV fluorescence (Cr³⁺, low-quenching chrysoberyl).
Synthetic colour-change spinel simulant: chalky blue-white SW fluorescence (Co²⁺ or
Ti⁴⁺) – completely different response, immediately diagnostic.
Synthetic alexandrite (Cr-doped) shows similar LWUV red to natural but lacks natural
inclusion types; growth features are definitive under magnification.
Sources
Qiu & Guo (2021)
Explaining Colour Change in Pyrope-Spessartine Garnets. Minerals 11(8), 865. DOI: 10.3390/min11080865. [VERIFIED] – Quantitative UV-Vis spectroscopic confirmation of dual transmission window mechanism; abstract explicitly describes the two transmittance zones and illuminant-dependence.
Schmetzer & Malsy (2011)
Alexandrite and colour-change chrysoberyl from the Lake Manyara alexandrite-emerald deposit in northern Tanzania. The Journal of Gemmology 32(5), 179–209. DOI: 10.15506/jog.2011.32.5.179. [VERIFIED] – Detailed optical characterisation of alexandrite; Cr³⁺ as chromophore confirmed.
Schmetzer, Bernhardt & Hainschwang (2013)
Titanium-bearing synthetic alexandrite and chrysoberyl. The Journal of Gemmology 33(5), 137–148. DOI: 10.15506/jog.2013.33.5.137. [VERIFIED] – Synthetic alexandrite diagnostics; Ti⁴⁺ absorption differences.
Read (2008)
Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224. [APPROXIMATE] – Crystal field context and named species survey.