Quarterly Crystal: Almandine-Pyrope Garnet in Diamond
The authors recently examined a collection of African diamond crystals that hosted various mineral inclusions. This Quarterly Crystal, chosen from that collection, is a 2.34 ct transparent near-colorless partially etched diamond octahedron measuring 7.71 × 7.39 × 4.92 mm, which came from the Republic of South Africa (figure 1). During routine testing, the near-colorless octahedron fluoresced a strong blue when exposed to long-wave (365 nm) ultraviolet radiation.
The diamond crystal hosts a transparent yellow-orange inclusion situated near its center. Laser Raman microspectroscopy identified this included crystal as an almandine-pyrope garnet. The yellow-orange color is typical of this type of inclusion. In addition to the garnet, light grayish green transparent birefringent omphacite crystals and opaque black sulfide and graphite crystals (some forming rosettes) were also present.
As inclusions in diamonds, transparent almandine-pyrope crystals are generally a medium to deep yellow-orange color with a vitreous luster. As shown in figure 2, such inclusions typically form as somewhat rounded protogenetic mineral crystals. Geologically, the presence of an almandine-pyrope garnet inclusion is an indicator that the host diamond formed in a rock type known as eclogite.
