Lab Notes Gems & Gemology, Summer 2014, Vol. 50, No. 2

Star Opal

Wasura Soonthorntantikul

6条スターのオパール
Figure 1. This 2.39 ct transparent opal with a light brownish yellow bodycolor possessed a six-rayed star. Photo by Lhapsin Nillapat.
Opal is best known for displaying play-of-color. It may also display asterism, though star opal has only been reported from Idaho, and a perfect six-rayed star is exceptionally rare (J.V. Sanders, “The structure of star opals,” Acta Crystallographica, Vol. A32, 1976, pp. 334–338). The Bangkok laboratory recently had the opportunity to examine a transparent star opal (figure 1). The 2.39 ct light brownish yellow cabochon displayed a distinct six-rayed star.

Standard gemological 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 gemological testing by energy-dispersive X-ray fluorescence (EDXRF) confirmed a silica-rich material with some additional trace elements, including aluminum and iron. All of these properties were consistent with opal.

Microscopic examination revealed large parallel planes with play-of-color 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.

Photomicrograph of star opal
Figure 2. This photomicrograph of the opal shows the intersection of large parallel planes with play-of-color, producing asterism. Photomicrograph by Charuwan Khowpong; magnified 15×.

Wasura Soonthorntantikul is a staff gemologist at GIA's Bangkok laboratory.