In the course of an investigation on refractory materials, in which the identification of minerals by their X-ray diffraction patterns had a prominent place, an interesting observation was made on a zircon of gem quality which was lent to us by the courtesy of Mr. B. W. Anderson.

It may be recalled that the existence of three allotropes has been postulated in order to explain the considerable variations of physical properties which are found when comparing different specimens of zircon. In particular, densities ranging from 3·95 to 4·86 have been observed, the density of zirconium silicate, calculated from X-ray measurements, being 4·71.