tiffanyite

from The Century Dictionary.

• noun The variety of diamond that appears stool-colored or steel-blue in color, possesses the property of emitting light in the dark after exposure to sunlight, electric light, or ultra-violet rays, and is unusually sensitive to radium, the Röntgen rays, polonium, and actinium. This property is due to the coloring-matter, evidently a phosphorescent hydrocarbon, or one of the rare earths.