Pentlandite Inclusion — A Mantle-Sourced Sulphide in Diamond
Pentlandite Inclusion — A Mantle-Sourced Sulphide in Diamond
A nickel-iron sulphide phase whose presence inside a diamond is a marker of formation in the upper mantle
Pentlandite inclusion refers to a small, opaque, metallic-grey to brassy-yellow inclusion of pentlandite — a nickel-iron sulphide of formula (Fe,Ni)9S8 — observed inside a diamond host. Pentlandite is one of several sulphide phases routinely catalogued among diamond's mineral inclusions, and its appearance is significant principally as a marker of the host diamond's formation environment in the upper mantle.
What it tells the gemmologist
Sulphide inclusions in diamond are documented in the Gübelin Photoatlas of Inclusions in Gemstones and in the GIA reference literature. Pentlandite is one of the iron-nickel-copper-sulphur phases — alongside pyrrhotite, chalcopyrite, and the more general monosulphide solid solution — that crystallise in the host diamond from sulphide melts present in the lithospheric mantle. The chemistry of these sulphide inclusions is one of the principal lines of evidence used by mantle geochemists to reconstruct the conditions under which a given diamond formed: the sulphide isotope ratios, the trace element partitioning, and the relative proportions of the iron and nickel sulphide species can constrain the temperature, pressure, and oxidation state of the diamond's source region.
Appearance and identification
Pentlandite inclusions are typically small, often submillimetric, and appear under magnification as opaque metallic grains with a slight brassy cast. They are surrounded in many cases by a black radial halo of stress fractures, the result of differential thermal contraction between the sulphide inclusion and the surrounding diamond as the host stone rose to the surface. Identification is by Raman spectroscopy and by electron microprobe analysis where sample preparation permits.
In the trade
For a cut diamond in the consumer market, pentlandite inclusions are clarity features that affect the GIA clarity grade in the same way as any other inclusion of similar size and position. They are uncommon in commercial-grade material and are generally too small to materially affect a stone's appearance. Where pentlandite inclusions become commercially relevant is in the high-end provenance market, where laboratories increasingly publish inclusion-suite documentation alongside grading certificates for stones of significant size or value. A pentlandite inclusion, properly characterised, is part of the natural-origin signature that distinguishes a mantle-formed diamond from any synthetic counterpart.