Lab Notes Gems & Gemology, Fall 2021, Vol. 57, No. 3

Glass Imitation of Star Sapphire

Glass cabochons exhibit fixed stars across their domes.
Figure 1. Glass cabochons with weights ranging from 0.47 to 1.29 ct were obtained by the author. The stones displayed white six-ray stars across their domes, resembling star sapphire. Photo by Jessa Rizzo.

The market for sapphire simulants and synthetics is plentiful, as sapphire is likely the most popular of all colored stones. The high cost of natural, gem-quality sapphire means it is not obtainable for much of the world’s population. This has opened the doors of creativity to produce blue stones with high luster resembling the natural material. Recently, the author obtained a parcel of blue cabochons with an unusual star pattern resembling star sapphire (figure 1). After a series of tests were performed, the material was identified as manufactured glass.

Cross section of the glass cabochon reveals flow lines.
Figure 2. A vertical cross section of the glass cabochon showcases the internal structure and diagnostic flow lines imitating star sapphire. Photo by Nathan Renfro.

Gemological investigation revealed an average specific gravity of 2.46 and a refractive index of 1.40. The cabochons were inert to long-wave ultraviolet radiation but exhibited strong chalky blue fluorescence under short-wave ultraviolet radiation. Observation under a microscope revealed gas bubbles shallow to the surface within the blue regions of the stones and small conchoidal fractures along the girdles. One cabochon was cut in half vertically for further scientific investigation (figure 2). This cross section uncovered a large core of white glass raised to the surface of the dome in a star pattern. Flow lines were also visible within the white glass core. Finally, a thin layer of blue glass around the perimeter of the cabochon was seen in the cross section, creating the bodycolor of the stone (again, see figure 2).

Asterism is an optical phenomenon that can be defined as a star-shaped concentration of reflected or refracted light from inclusions within a gemstone cut en cabochon. Genuine asterism is also mobile if the stone and/or light source is moved. These glass imitation star sapphires, however, do not exhibit genuine asterism, as their stars were simply molded into shape and are fixed in place.

Glass imitations span the gamut of gemstones. Faceted transparent colorless glass is one of the oldest and simplest diamond simulants. Glass beads coated with pearlescent paint are a common pearl imitation, and colored glass of various opacities is capable of imitating almost any colored gemstone. Recent G&G articles on colored stone imitations include “Artificial glass imitating a Paraíba tourmaline” (Winter 2020 Lab Notes, pp. 518–520), “Greenish blue glass imitating gem silica” (Summer 2020 Gem News International, pp. 314–315), and “Glass bangles” imitating jadeite and nephrite (Spring 2019 Lab Notes, pp. 93–94).

Britni LeCroy is a staff gemologist at GIA in Carlsbad, California.