Alexandrite is a variety of chrysoberyl, aluminium beryllate, coloured by chromium. What makes it exceptional is not merely that it changes colour, but the specific way in which the change occurs and what it reveals about the relationship between light, absorption and human vision. Every gemmologist who understands the mechanism finds the stone more interesting as a result.
The physics of the colour change
Chromium absorbs light in two ranges: one centred around 580nm (orange-yellow) and one around 410nm (violet). The transmission window it leaves open falls between these two absorption bands, in the green and red portions of the spectrum. Daylight and fluorescent light are rich in short wavelengths, so the transmitted light is perceived as green. Incandescent and candlelight are rich in long wavelengths, shifting the transmitted light into the red. Human vision also has a natural sensitivity shift between conditions, the Purkinje effect, that amplifies the perceived colour change. The result is a stone that appears genuinely different under different light sources, not simply lighter or darker.
Quality: what makes a fine alexandrite
The quality of the colour change is the primary value driver. An ideal alexandrite shows a dramatic, saturated shift, vivid green in daylight, vivid red in incandescent light, with no muddiness, brownishness or muted intermediate hues. In practice, most alexandrite shows a less than perfect shift: bluish-green to brownish-red, or olive-green to brownish-purple. Fine colour change with good saturation in both positions is exceptional. Clarity matters secondarily, but the nature of alexandrite means that heavily included stones still have strong collector interest if the colour change is exceptional.
Russian alexandrite: the historical benchmark
The Ural Mountains of Russia produced the original alexandrite discovery in the 1830s, reportedly discovered on the same day as Tsar Alexander II’s birthday, giving the stone its name and its association with the Russian imperial family. Ural alexandrite shows the most dramatic colour change of any major deposit and has a distinctive inclusion scene. The deposit is essentially exhausted, and original Ural alexandrite of notable size commands extreme premiums. Any stone claimed as Russian Ural requires laboratory confirmation of origin.
Contemporary sources
Brazil has produced significant alexandrite since the 1980s, particularly from Minas Gerais. Brazilian material ranges from excellent to mediocre in colour change; the finest Brazilian stones rival Russian quality. Sri Lanka yields alexandrite in larger sizes with less dramatic colour change, useful for cat’s-eye alexandrite. Madagascar, India and East Africa are smaller contemporary producers. Laboratory-grown alexandrite is widely available and inexpensive; it shows a near-perfect colour change because the growth conditions can be controlled, which paradoxically makes it an easy identification target for experienced gemmologists who are suspicious of a stone showing an unusually strong shift.
Identification pitfalls
Alexandrite is imitated by synthetic corundum and synthetic spinel with colour-change properties. The imitations do change colour, but the shift tends toward purple rather than green-to-red. Refractive index separates the imitations immediately: alexandrite reads 1.746–1.755, distinctly higher than corundum or spinel. A refractometer reading eliminates all common imitations in seconds. The more common identification challenge is distinguishing natural from laboratory-grown alexandrite of the same chemistry; spectroscopic analysis and inclusion examination are necessary for that determination.