The Russian alexandrite effect is the dramatic shift in apparent body colour from green under daylight illumination to red or purplish-red under incandescent light, observed at its finest in alexandrite from the Ural Mountains in Russia and used as the benchmark for the colour-change phenomenon as a whole. The effect is the optical signature of the species and the principal driver of value in fine alexandrite; its strength, the cleanliness of the two end-member colours, and the absence of intermediate brown or olive overtones distinguish the best stones from material whose colour change is weaker or muddier.

Physical basis

Alexandrite is a chromium-bearing variety of chrysoberyl, BeAl₂O₄, in which a small percentage of the aluminium atoms in the structure are replaced by chromium. The substituted chromium creates absorption bands in the visible spectrum that flank the eye's region of peak sensitivity near 555 nanometres, transmitting two distinct windows: a green-blue window in the 480 to 520 nanometre range and a red window above 650 nanometres. Daylight, which is rich in shorter wavelengths, drives the green window into prominence; incandescent light, which is rich in longer wavelengths, drives the red window. The eye, integrating across the transmitted spectrum, reads the stone as green under one source and red under the other.

The strength of the colour change depends on the cleanliness of the two transmission windows and on the relative balance of chromium and competing chromophores, particularly iron. Iron suppresses the saturation of both windows and adds a brownish or olive overtone, so stones with low iron content — Ural alexandrite among them — produce the cleanest and most dramatic colour change. Stones with higher iron content — much Brazilian, East African, and Indian material — produce a weaker or muddier change, in which the green is bluish-green or olive and the red is closer to wine-purple.

Quantification

Modern laboratories quantify the colour change as a percentage figure derived from the spectral and colorimetric measurement of the stone under standardised daylight (D65) and incandescent (A) illuminants. A stone described as showing 100 percent colour change appears completely green in daylight and completely red under incandescent light; values from 60 to 100 percent are typical for fine commercial alexandrite, with Ural material commonly above 80 percent and the finest Ural pieces approaching the maximum. The percentage figure is now standard on laboratory reports from GIA, Gübelin, SSEF, and AGL, and is the most useful single number for comparing the quality of the colour change across stones of different origin.

Visual evaluation

In bench evaluation, the trade tests the alexandrite effect by alternating daylight (or daylight-equivalent fluorescent) and incandescent illumination and observing the change. The cleanest stones produce a near-instant flip in apparent colour as the light source changes; weaker stones show a gradual drift through brown or olive intermediate colours. The size and proportions of the cut stone also affect the perceived strength of the effect, with shallow stones leaking light through the pavilion and weakening both end-member colours. Faceting design for alexandrite emphasises pavilion depth and a brilliant-cut crown to maximise return through the visible spectrum.

In the trade

For dealers and laboratories, the strength of the alexandrite effect is the principal determinant of value above the basic factors of weight, clarity, and proportion. The Russian alexandrite effect — the cleanest, most dramatic version of the phenomenon, named for the type-locality material — is the standard against which alexandrite from Brazil, Sri Lanka, Tanzania, India, and Madagascar is compared, and the trade premium for stones approaching the Russian benchmark is substantial regardless of geographic origin. See also alexandrite, Russian alexandrite, and chrysoberyl.

Further reading

• GIA Gems & Gemology — colour change phenomena
• International Gem Society — alexandrite
• ICA — alexandrite reference