June 27, 2014
Standard gemmological testing gave a spot RI reading of 1.43 and a hydrostatically calculated SG of 2.10. When exposed to ultraviolet radiation, the stone fluoresced strong bluish white under long-wave and weak bluish white under short-wave UV. It phosphoresced green after exposure to long-wave UV. Advanced gemmological testing by energy-dispersive X-ray fluorescence (EDXRF) confirmed a silica-rich material with some additional trace elements, including aluminium and iron. All of these properties were consistent with opal.
Microscopic examination revealed large parallel planes with play-of-colour intersecting to form a hexagonal pattern (figure 2). This is responsible for producing the six-rayed star. This unusual stone serves as a reminder that, unlike asterism in other gem materials, the star in opal is caused by diffraction of light from faults or imperfections in the packing arrangement of silica spheres.
About the Author
Wasura Soonthorntantikul is a staff gemmologist at GIA's Bangkok laboratory.