gems, gem testing, gemology, field gemology, polariscope, interference figure

Emmanuel Fritsch recently received a synthetic berlinite crystal, donated by Dr. Robert Shannon of the E. I. Dupont Central Research Laboratory and grown by Bruce H.T. Chai at the University of Central Florida. Berlinite, a naturally occurring aluminum phosphate (AlPO₄), is structurally isomorphous with quartz; both crystallize in the hexagonal (trigonal) system and are optically active, occurring in left- and right-handed varieties. When viewed between crossed polarizers, untwinned berlinite displays the “bull’s-eye” optic figure (figure 9), previously described in gemological literature as a feature unique to quartz. This observation suggests that the “bull’s-eye” figure may, in fact, be characteristic of all optically active uniaxial crystals.