Light scatters within the stone, and a blue-white glow rises to the surface. This is the Schiller effect—caused by light interfering with ultra-thin layers of feldspar.

Found primarily in India, Sri Lanka, and Myanmar. In India, it has long been regarded as solidified moonlight. The Roman naturalist Pliny recorded that its glow waxed and waned with the phases of the moon.

No scientific basis for any of it. But the shifting light was convincing enough. During the Art Nouveau period, René Lalique's frequent use in jewellery established its standing in the West. The belief that moonlight could be held inside a stone—that beautiful error keeps it in a place apart.