Periclase Inclusion — A Deep-Mantle Phase Trapped Inside Diamond
Periclase Inclusion — A Deep-Mantle Phase Trapped Inside Diamond
A magnesium oxide mineral whose presence inside a diamond is a marker of formation in the lower mantle, well below typical lithospheric depths
Periclase inclusion refers to a mineral inclusion of periclase — magnesium oxide, MgO — observed inside a diamond host. Periclase is rare at the Earth's surface and the few terrestrial occurrences are entirely the product of metamorphism of magnesium-rich carbonate rocks. Its appearance inside a diamond is geologically consequential: periclase is one of the high-pressure mineral phases used by mantle petrologists to identify diamonds that crystallised at depths exceeding the typical 150 to 200 kilometres of lithospheric diamond formation. Periclase-bearing diamonds are sublithospheric in origin, having grown in the transition zone or the lower mantle and risen to the surface through kimberlite eruption.
The geology
The Earth's lower mantle is dominated mineralogically by ferropericlase — a solid solution between periclase (MgO) and wüstite (FeO) — alongside the higher-pressure polymorph of magnesium silicate known as bridgmanite. A diamond that crystallises in the lower mantle is exposed to this assemblage, and any inclusions trapped during diamond growth carry the lower-mantle mineral chemistry. Periclase, sometimes with elevated iron content where the inclusion approaches ferropericlase composition, is the most commonly observed lower-mantle mineral in diamonds, in part because it is more stable on ascent than the silicate phases that decompose as pressure drops.
The standard reference for sublithospheric diamonds and their inclusion suites is the GIA's research group, which has published extensively on the topic, alongside scientific journals including Nature, Lithos, and the GIA's own Gems & Gemology. The CLIPPIR diamonds — the population of large, type IIa diamonds that include the Cullinan, the Lesotho Promise, and the Constellation — are predominantly sublithospheric and frequently carry periclase or its decompression products as inclusions.
Identification
Periclase inclusions appear under magnification as small, opaque or translucent grains, often surrounded by a stress halo from differential thermal contraction during ascent. Identification is by Raman spectroscopy and by electron microprobe analysis, with the iron content of the inclusion (the ferropericlase composition) often providing additional information about the source region. The presence of an inclusion of pure or near-pure periclase in a diamond is itself diagnostic: there is no realistic alternative scenario for the formation of this mineral assemblage in the diamond context.
In the trade
For the consumer market, periclase inclusions are clarity features and are graded as such by GIA and other laboratories. They are uncommon and are concentrated in the population of large, exceptional diamonds where laboratory documentation goes well beyond standard grading. A diamond with documented periclase inclusions has a story — sublithospheric origin, lower-mantle formation, depth of perhaps 660 kilometres or more — that some clients in the high-end market value as part of the stone's narrative provenance. Whether this translates into a measurable price premium varies by the specific stone and the market context.