Lab Notes Gems & Gemology, Spring 2021, Vol. 57, No. 1

Rare Faceted Hexagonite

A 3.27 ct faceted hexagonite.
Figure 1. This 3.27 ct pinkish purple hexagonite is the largest example examined by the authors. Photo by Robert Weldon; courtesy of Vance Gems.

A saturated pinkish purple hexagonite (figure 1) was recently examined at the Carlsbad laboratory. Standard gemological testing revealed a refractive index of 1.599–1.629 and a specific gravity of 2.97 obtained hydrostatically. Microscopic examination showed needle-like inclusions and iridescent cleavage cracks (figure 2).

This faceted hexagonite contained iridescent cleavage cracks.
Figure 2. Iridescent cleavage cracks and needle-like structures were present in the faceted hexagonite. Photomicrograph by Nathan Renfro; field of view 3.50 mm.

When observed with a dichroscope, pleochroic colors of pink, orangy pink, and violet were observed. These results were consistent with the mineral tremolite. The pinkish purple variety of tremolite is recognized as hexagonite, which owes its color to manganese. Hexagonite was once thought to be a hexagonal form of tremolite, which is how its name was derived, but was later confirmed to be monoclinic (Summer 1985 Gem News International, p. 110).

This particular stone was faceted by Bill Vance of Vance Gems, who sourced the rough that was reportedly from the original type locality, the Balmat-Edwards zinc mining district in St. Lawrence County, New York. Hexagonite is also found at the town of Fowler in St. Lawrence County. The saturated pinkish purple color and large size make this rare stone an exceptional example of this material. While very few faceted hexagonite gems have been tested at GIA, this 3.27 ct stone is the largest one examined by the authors.

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