A gemstone that looks blue-green beside a window can look purplish-red under an incandescent lamp. Move it back and the green returns. The crystal has not acquired a new pigment: a different mixture of incoming wavelengths has changed the light reaching your eyes. Alexandrite is the classic example, but several other gemstone varieties show related shifts.
The short answer
One crystal, two competing color windows
Alexandrite filters light through absorption bands that leave blue-green and red transmission regions. Daylight and incandescent light supply those regions in different proportions. The surviving light, interpreted by an adapting visual system, can therefore produce distinctly different colors. Gemologists call this illuminant-dependent behavior the alexandrite effect.
How chromium shapes alexandrite's spectrum
Alexandrite is chromium-bearing chrysoberyl. Small amounts of chromium occupy sites normally held by aluminum in the crystal structure. These chromium ions, written Cr³⁺, absorb selected wavelengths rather than removing all visible light equally. Their surroundings in the lattice help determine the energies they absorb, so chromium does not give every mineral the same color.
A broad absorption band centered around 580 nanometers removes much of the yellow to yellow-orange region. Absorption toward the violet end also shapes the spectrum. Between these losses lie two useful transmission windows: one in the blue-green region and another in the red. These are broad regions, not two perfectly isolated colors or sharp openings in a filter.
In a stone with a pronounced change, the two windows can make closely competing contributions to perceived color. That balance is why changing the illumination matters so much. The source spectrum matters beyond its everyday label, too: two bulbs sold as warm white can disagree, and their effects on colored materials need not match.
Why daylight and incandescent light tip the balance
A light's spectral power distribution describes how much energy it supplies at each wavelength. Compared with incandescent light, daylight generally supplies more short-wavelength visible light relative to red. An incandescent filament produces a continuous spectrum weighted more heavily toward longer wavelengths; candlelight is similarly warm. Neither source is just a single color.
The light leaving the gemstone depends on the incoming spectrum multiplied, wavelength by wavelength, by its transmission. Under daylight, the blue-green window can contribute enough to make alexandrite appear green or blue-green. Under incandescent light or candlelight, the stronger relative red contribution can shift its appearance toward red or purplish-red. The absorption pattern itself need not change.
This is a simplified picture of a faceted stone, where rays also reflect internally and travel different distances. Chromium concentration, crystal orientation, and light path length affect the result. Some specimens change strongly; others retain mixed or muted hues. Daylight also varies with weather, time, and surroundings, so descriptions name typical appearances rather than guaranteed color coordinates.
Your eyes are part of the comparison
The eye does not read a spectrum as a row of separate wavelength values. Its three cone classes have overlapping sensitivities, and the visual system compares their responses. It also adapts to illumination: a white surface can continue to look approximately white when you move from daylight into a warmly lit room. This color constancy is useful, but incomplete.
Alexandrite's unusual spectral balance interacts with those responses and with chromatic adaptation. The stone can change its apparent hue relative to its surroundings, rather than merely sharing the warm cast that falls across everything nearby. Background color, viewing conditions, and the observer's adaptation therefore belong in any careful comparison.
The effect illustrates the illuminant dependence also encountered in metamerism. Strictly, metamerism involves different spectra producing a color match under specified conditions; two objects that match under one lamp may separate under another. A single gemstone changing appearance is not itself that two-object match, but it exposes the same limitation: perceived color cannot be assigned from the material alone.
Other stones and effects worth distinguishing
Some sapphires, garnets, and spinels also show color change, although their hue pairs and strength vary. Trace elements such as vanadium and chromium can contribute, depending on the mineral and its composition. These coloring agents are called chromophores. Sharing a color-change effect does not mean sharing chrysoberyl's chemistry, chromium environment, or exact absorption spectrum.
Changing the lamp is also different from turning the stone. Alexandrite can show pleochroism: different colors along different crystal directions under the same illumination. Keep orientation steady when testing the alexandrite effect so those two variables do not become confused. Neither effect is the same as a diamond's small rainbow flashes, which arise from dispersion as different wavelengths bend by different amounts.
Check the gemstone under two lights
Start beside a window in indirect daylight, then view the same stone under an incandescent lamp. Use one source at a time, keep the viewing angle similar, and choose a neutral background. Give your eyes time to settle in each setting. Look at the overall body color rather than an isolated bright reflection, and note both hues instead of expecting every specimen to turn pure green and pure red.
A warm LED is not automatically a substitute for incandescent light: similar-looking lamps can distribute energy differently across the spectrum. Mixed window and room lighting can soften the comparison. Photographs add camera white balance and screen rendering to the chain, so a pair of images is less reliable than observing the stone directly under known conditions.
This two-light check reveals behavior, not identity or origin. A color change alone cannot establish that a stone is natural alexandrite; synthetic materials and other gems can show similar effects. Identification requires gemological testing. For everyday viewing, the useful habit is simpler: record the light along with the color, because both describe what you saw.