Exquisite Opal Strand
The Carlsbad laboratory recently received for identification a strand of 28 graduated round gray beads displaying spectacular play-of-color (see above). At the time of inspection, the strand weighed 115.5 grams total. Standard gemological properties were consistent with opal. All 28 beads had a spot RI reading of approximately 1.39 and fluoresced very weak yellow under short-wave UV and weak yellow to long-wave UV. No indications of dye, color treatment, clarity enhancement, or coating were present. All of the beads showed a natural play-of-color phenomenon under microscopic examination. A single drop of water was placed on the surface of each bead and observed under direct transmitted light in brightfield mode in the microscope. Each bead absorbed the water, revealing them to be porous. Therefore, it was concluded that all 28 beads were natural hydrophane opal with natural color. Crazing was not observed in any of them.
Hydrophane opals are porous enough to absorb water, which can cause a change in appearance and a temporary increase in weight. It is therefore not advisable to immerse them in water (Fall 2013 Lab Notes, pp. 175–176).
This opal strand was remarkable due to the unusually bright and vibrant spectral play-of-color seen evenly across all 28 beads. Play-of-color in precious opal is caused by light interacting with stacked submicroscopic silica spheres. Light waves bend as they pass between these spheres, causing the light to diffract into the spectral colors of the rainbow. The resulting color is determined by the size of the spheres. For example, approximately 0.1 micron spheres produce a violet color and approximately 0.2 micron spheres display a red color (https://www.gia.edu/opal-description).
