Spurrite

A 19.68 ct gray-purple spurrite. — Figure 1. A 19.68 ct gray-purple spurrite that was submitted to GIA’s Carlsbad lab. Photo by Robison McMurtry.

Recently, a 19.68 ct gray-purple stone (figure 1) was submitted to the Carlsbad laboratory for identification. The square cabochon was semitranslucent and had wispy white features throughout.

Raman spectrum of the spurrite. — Figure 2. Raman spectroscopy identified the gray-purple stone as spurrite.

It had a refractive index of 1.68–1.64 and a hydrostatic specific gravity of 3.00. Raman spectroscopy identified it as spurrite (figure 2), which aligned with the gemological properties already measured. Spurrite is a nesosilicate with a chemical composition of Ca₅(SiO₄)₂CO₃ (J.W. Anthony et al., Handbook of Mineralogy, Vol. 2, Mineral Data Publishing, 1995). Analysis by energy-dispersive X-ray fluorescence (EDXRF) showed high amounts of calcium and silicon.

Magnetite inclusion in spurrite. — Figure 3. Magnetite, identified using Raman spectroscopy, breaks the surface of the spurrite. Photomicrograph by Nicole Ahline; field of view 4.79 mm.

Raman spectrum of the magnetite inclusion. — Figure 4. Raman spectroscopy identified the inclusion in the spurrite as magnetite.

Spurrite occurs in high-temperature thermal metamorphism along the contact between carbonate rock and mafic magma (Anthony et al., 1995). A common inclusion found within spurrite is black metallic magnetite crystals. Raman spectroscopy identified the metallic opaque inclusions that broke the surface (figure 3) as magnetite (figure 4), further evidence that this stone was a spurrite.

Encountering stones that require more research and a little detective work, such as this spurrite, can enhance one’s curiosity about gemology.

Nicole Ahline is a senior staff gemologist at GIA in Carlsbad, California.

Recently, a 19.68 ct gray-purple stone (figure 1) was submitted to the Carlsbad laboratory for identification. The square cabochon was semitranslucent and had wispy white features throughout.