Gem News International Gems & Gemology, Spring 2015, Vol. 51, No. 1

Dumortierite In Rock Crystal Quartz

Nathan Renfro, Ziyin Sun, and John Koivula

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Quartz with dumortierite inclusions
Figure 1. This group of rock crystal quartz from Brazil shows vibrant blue dumortierite inclusions. The largest rough crystal weighs 139.79 ct, and the faceted stone weighs 15.47 ct. Photo by Kevin Schumacher.
Dumortierite, which commonly occurs as a blue borosilicate mineral, is of gemological interest when present in quartzite, making an attractive blue ornamental material. Recently examined by GIA’s Carlsbad laboratory were several examples of dumortierite inclusions in rock crystal quartz (figure 1) provided by Luciana Barbosa (Asheville, North Carolina). According to Mrs. Barbosa, the material is reported to be from the Brazilian state of Bahia, in the Serra do Espinhaço Range near the Vaca Morta quarry.

Dumortierite clusters
Figure 2. Clusters of vibrant blue acicular dumortierite were observed in this rock crystal quartz. Note that some dumortierite crystals are brownish due to epigenetic mineral staining along the interface of the quartz host and the dumortierite inclusions. Photomicrograph by Nathan Renfro; horizontal field of view 7.38 mm.
Much of the material examined by the authors showed phantom planes and clusters of acicular needles with a vibrant blue coloration (figure 2). Polarized light revealed very strong blue to colorless pleochroism in the blue mineral inclusions. Raman analysis confirmed the identity of the needles as dumortierite. Also observed in one sample were colorless acicular crystals on a phantom plane, which were also identified by Raman as dumortierite (figure 3).

Dumortierite needles in quartz
Figure 3. Unusual pale blue to colorless needles of dumortierite were also observed in the rock crystal quartz. Photomicrograph by Nathan Renfro; horizontal field of view 2.24 mm.
Blue color in dumortierite has previously been reported to be caused mainly by Fe2+-Ti4+ charge-transfer (see http://minerals.gps.caltech.edu/FILES/Visible/dumortierite/Index.htm). The chemical composition of the blue and colorless dumortierite inclusions was analyzed by LA-ICP-MS to look for any prominent differences that might explain the variance in observed color. The most significant difference was in the magnesium content, which was almost 10 times higher in the colorless dumortierite inclusions (1070 ppma, compared to 127 ppma Mg in the blue dumortierite). While more research is needed to fully understand the role this higher magnesium content has on color, the authors speculate that the titanium preferentially charge-compensates with magnesium instead of iron. If there is not enough excess titanium relative to magnesium, it may not be possible for titanium to pair with divalent iron, and this would prevent the formation of blue color.

These are the first examples of prismatic blue and colorless dumortierite inclusions in rock crystal quartz we have encountered.

Nathan Renfro is analytical manger of gem identification, Ziyin Sun is a staff gemologist, and John Koivula is an analytical microscopist, at GIA's laboratory in Carlsbad, California.