Pronunciation

ih-LOO-mi-nant

A specified light source used to evaluate gemstone color, defined by its color temperature in Kelvin and its spectral power distribution. Standard illuminants in gemology include D65 (average daylight, 6500 K), A (incandescent, 2856 K), and candlelight (~1850 K). Color-change gems like Zultanite display different colors under different illuminants because each illuminant emphasizes different wavelengths in the gem’s transmission spectrum.