Color Change

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Glossary Term

Color Change

Diaspore · Zultanite
Fact-checked 2026-04-27

By Dr. Elif Demir Mineralogy Editor

Pronunciation
C:/Program Files/Git/ˈkʌlər tʃeɪndʒ/

Color change (gemology) — also called the alexandrite effect — is the appearance of distinctly different colors in the same gemstone when viewed under different illuminants, typically daylight versus incandescent or candlelight. The phenomenon arises from selective absorption: trace ions in the crystal lattice absorb specific wavelengths, and the residual transmitted light differs between cool-spectrum and warm-spectrum illuminants.

How it works

Color change is not a property of the crystal itself changing. The crystal’s absorption spectrum is fixed by its atomic structure and trace-element content. What changes is the spectrum of the incoming light. Daylight (CIE Illuminant D65) is rich across the visible spectrum and weighted toward blue-green; incandescent tungsten light (CIE Illuminant A) is heavily weighted toward red. The same crystal, sampling these different spectra, transmits residual light in different perceptual color regions.

Common color-change gems

  • Alexandrite (chrysoberyl variety) — the type material; green to red shift driven by chromium.
  • Color-change diaspore (Zultanite, Csarite, Ottomanite, Turkizite) — sage-green to raspberry-pink shift driven by iron and chromium.
  • Color-change garnet (pyrope-spessartine) — typically blue-green to red-purple, multiple producing localities.
  • Color-change sapphire — usually blue-violet to purple-red, vanadium-driven.
  • Color-change fluorite — less commercially significant; varies widely.
  • Pleochroism — different colors viewed along different optical axes under the same light. A property of the crystal, not the illuminant.
  • Asterism — the star-effect from aligned inclusions; not a color change.
  • Adularescence — moonstone’s moving sheen; the color of the sheen can shift with viewing angle but is not color change in the gemological sense.

Diagnostic test

Place the stone under daylight (or daylight-balanced fluorescent), then under incandescent or candlelight. If the hue shifts to a substantially different color region, that is color change. If the hue brightens, darkens, or shifts saturation but stays in the same color family, it is not color change in the gemological sense.

See also: pleochroism, Mohs scale.

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