Lab Notes Gems & Gemology, Summer 2020, Vol. 56, No. 2

Hemimorphite Resembling Paraíba Tourmaline

Electric blue 5.61 ct hemimorphite.
Figure 1. This 5.61 ct electric blue cabochon was identified as hemimorphite. Photo by Nuttapol Kitdee.

A 5.61 ct semitransparent blue cabochon (figure 1) was submitted to the New York laboratory for a Paraíba tourmaline origin report because of its electric blue color and its similar RI and internal features. Microscopic observation revealed parallel tubes, fluid inclusions, and white granular flake-like inclusions. The stone’s optic character could not be determined due to the cabochon shape and abundance of inclusions, and only an approximate RI of 1.61–1.63 with a birefringence of 0.02 could be obtained. Specific gravity was 3.45, which is inconsistent with tourmaline, and the fluorescence reaction was medium blue under short-wave and inert under long-wave UV radiation. We confirmed its identity by Raman spectroscopy, which showed a match with hemimorphite (, as shown in figure 2.

Raman spectrum of this hemimorphite matches with the known reference spectrum.
Figure 2. Raman spectra of a 5.61 ct semitransparent blue cabochon that matched with hemimorphite in the RRUFF database (no. R070536).

Hemimorphite is a zinc silicate, Zn4Si2O7(OH)2·H2O; the purest form is white or colorless. Impurities cause different colors, such as copper (Cu2+) for a bluish and greenish tint, ferrous iron (Fe2+) for green, and ferric iron (Fe3+) for brown. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) composition analysis revealed that this stone contained no iron but did contain copper, which produced the electric blue color.

Sudarat Saeseaw is senior manager of colored stones identification at GIA in Bangkok.