The common pale-blue color of celestite bleaches at about 200°C and reappears upon X-ray irradiation. A thermoluminescence maximum for blue celestite also occurs around 200°C. The thermal stability of the blue color (as measured by the bleaching time at 190°C) was found to be proportional to potassium content. Electron spin resonance data indicate the presence of , , , and O− in blue celestite crystals. These paramagnetic hole centers presumably arise by the action of ionizing radiation on , which releases energetic electrons that can decompose other groups. The centers , , and O− absorb in the visible, producing the blue color, while absorbs in the ultraviolet. The color-producing centers are stabilized in the celestite lattice by the presence of trace components, primarily K.+ substituting for Sr2+.
The color in two specimens of orange celestite was correlated to the presence of copper, probably as Cu+, in the order of 50 ppm.