Micro-World Gems & Gemology, Spring 2018, Vol. 54, No. 1

Quarterly Crystal: Cr-Diopside in Diamond

Bright green chromium diopside inclusion in diamond octahedron.
Figure 1. Measuring 7.59 mm in largest dimension and weighing 1.84 ct, this partially etched diamond octahedron contains a bright green inclusion of chromium diopside. Photo by Robison McMurtry.

Brightly colored mineral inclusions in diamonds are rare. Some examples of these rarities are dark blue kyanite, yellowish orange almandine-pyrope, and deep purplish red pyrope garnet. This year’s first Quarterly Crystal offering expands on this bright inclusion theme with a transparent colorless partially etched octahedron from the Kimberley mine in South Africa. Shown in figure 1, the diamond measures 7.59 × 6.59 × 4.79 mm and weighs 1.84 ct.

Situated along the edge of one of the octahedral planes, the diamond crystal plays host to a bright green transparent elongated mineral inclusion. Laser Raman microspectrometry was used to identify this included crystal as diopside, and the bright green color results from a trace amount of chromium.

Rounded, etched chromium diopside inclusion in diamond.
Figure 2. Rich green transparent crystals of chromium diopside are very uncommon inclusions in diamond. The example shown here has a typical rounded, etched habit. Photomicrograph by Nathan Renfro; field of view 1.80 mm.

As a diamond inclusion, chromium diopside is a medium to deep green transparent mineral with a vitreous luster. As shown in figure 2, it typically forms as rounded protogenetic mineral grains. Geologically, the presence of a Cr-diopside inclusion in a diamond is an indicator that the host diamond formed in a rock type known as peridotite.

John I. Koivula is analytical microscopist at GIA in Carlsbad, California.