Micro-World Gems & Gemology, Spring 2019, Vol. 55, No. 1

Grandidierite Inclusions in Sapphires

Lath-like grandidierite mineral inclusion in sapphire.
Figure 1. Lath-like mineral inclusions of grandidierite were seen in this sapphire at GIA’s Carlsbad laboratory. Photomicrograph by Nathan Renfro; field of view 2.61 mm.

Recently the authors independently encountered an inclusion that has not been previously reported in sapphire. Both stones contained colorless crystals that were identified by Raman analysis as the mineral grandidierite. The sapphire examined by author MH in Carlsbad, California, contained colorless lath-like inclusions (figure 1). The sapphire examined by author EBH in Bangkok contained a colorless crystal that reached the surface of the sapphire host and displayed a duller luster in reflected light, and birefringent interference colors when examined using cross-polarized light (figure 2). Both observations are consistent with what one would expect for grandidierite.

A colorless grandidierite crystal in sapphire (left) examined in reflected light (center) and cross-polarized light (right).
Figure 2. A colorless grandidierite crystal was seen in a sapphire examined by author EBH at Lotus Gemology in Bangkok (left). In reflected light, the grandidierite inclusion showed a duller luster than the sapphire host (center) and cross-polarized light revealed birefringent interference colors (right). Photomicrographs by E. Billie Hughes; field of view 1.7 mm.

Grandidierite, named after French naturalist Alfred Grandidier (1836–1921), is an extremely rare orthorhombic Mg-Fe aluminous borosilicate with the formula (Mg,Fe)Al3(BO3)(SiO4)O2. The mineral is described as bluish green to greenish blue; the blue color increases with Fe content. It is transparent to translucent with a pale yellow to colorless, greenish blue, and blue trichroism. Since its discovery, grandidierite has been found as a rare accessory mineral in aluminous boron-rich pegmatite; in aplite, gneiss, and crystalline rock associated with charnockite; and in rock subjected to local high-temperature, low-pressure metamorphism (contact aureoles and xenoliths). To the authors’ knowledge, these are the first observations of grandidierite as an inclusion in sapphire.

Maxwell Hain is a staff gemologist at GIA in Carlsbad, California. E. Billie Hughes is a gemologist at Lotus Gemology in Bangkok.