Micro-World Gems & Gemology, Spring 2021, Vol. 57, No. 1

Unusual Fibrous Inclusions in Blue Opal

Blue opal contains unusual curled fiber inclusions.
These unusual clusters of curled fibers were seen in a blue opal. Photomicrograph by Nathan Renfro; field of view 3.58 mm

The authors recently encountered an opal with a grayish blue bodycolor and weak play-of-color. The gem also contained some interesting fibrous inclusions (see above). The inclusions were generally scattered throughout a planar zone as curled bundles of chatoyant fibers. This type of opal appeared to be naturally colored and consistent in appearance and physical properties with opal from Sinaloa, Mexico (Fall 2011 Gem News International, p. 243). Raman analysis was unable to identify the inclusions, but a strong cristobalite signal was measured in areas with the fibers and without them, suggesting a volcanic origin (E. Fritsch et al., “Green-luminescing hyalite opal from Zacatecas, Mexico,” Journal of Gemmology, Vol. 34, No. 6, 2015, pp. 490–508). While the exact nature of these inclusions remains a mystery, this is the first time the authors have encountered this type of unusual coiled fibrous inclusion in blue opal.

Nathan Renfro is manager of colored stone identification, and Nicole Ahline is a senior staff gemologist, at GIA in Carlsbad, California.