Olivine Inclusion — A Mantle-Origin Marker in Diamond
Olivine Inclusion — A Mantle-Origin Marker in Diamond
Forsterite-rich olivine crystals trapped in diamond during mantle formation, diagnostic of peridotitic paragenesis
An olivine inclusion in diamond is a small crystal of forsterite-rich olivine — magnesium-iron silicate, (Mg,Fe)2SiO4 — that was trapped within the host diamond during diamond formation in the Earth's mantle, typically at depths of approximately 150 to 200 kilometres. Olivine inclusions are among the most common mineral inclusions in natural diamond and are diagnostic of the peridotitic paragenesis — the suite of mineral phases characteristic of mantle peridotite, the dominant rock type of the upper mantle. Their identification is one of the standard tools used by gemmologists and laboratories to confirm mantle origin, distinguish natural from synthetic diamond, and reconstruct the conditions of diamond formation.
Geological significance
Diamond crystallises in the mantle, in two principal mineral assemblages: peridotitic (associated with mantle peridotite, the olivine-pyroxene-garnet rock that dominates the upper mantle) and eclogitic (associated with eclogite, the high-pressure equivalent of basalt). The paragenesis is identified by the suite of inclusions: peridotitic diamonds contain forsterite-rich olivine, chromite, chrome-pyrope garnet, and chrome-diopside; eclogitic diamonds contain almandine-pyrope garnet, omphacite, and rutile. The two parageneses correspond to different mantle source regions and different histories of carbon supply to the diamond-forming environment.
Olivine inclusions in peridotitic diamonds are typically forsterite-rich (90 to 95 per cent forsterite component), reflecting the magnesium-rich character of mantle olivine and confirming the cratonic mantle source. The composition is documented in extensive electron-microprobe analyses summarised in the Gübelin Photoatlas of Inclusions and in published GIA research.
Appearance under magnification
Olivine inclusions in diamond appear as rounded to angular crystals, often a fraction of a millimetre across, with characteristic colourless to pale green tinge when fresh and unaltered. Many show partial alteration to serpentine or other secondary minerals, particularly along fractures connecting the inclusion to the surface. The inclusion-host interface frequently shows characteristic stress halos — the diamond surrounding the inclusion is under residual compressive stress from the differential thermal contraction of the two phases on cooling from formation temperature.
Diagnostic value
The presence of olivine inclusions is among the strongest diagnostic markers of natural mantle origin in diamond. Synthetic diamonds — both high-pressure-high-temperature and chemical-vapour-deposition synthetic — do not contain olivine inclusions. Laboratory-grown diamond inclusions, when present, are characteristic of the synthesis chemistry: metallic flux residues in HPHT diamond, graphite or void-like inclusions in CVD material. The unambiguous identification of an olivine inclusion in a stone tested for natural-versus-synthetic origin therefore constitutes positive evidence of natural origin.
For peridotitic diamonds, olivine inclusions also help reconstruct the depth and conditions of diamond formation. Combined with garnet, chromite, and clinopyroxene inclusions, the mineral assemblage allows estimation of pressure and temperature at the time of trapping using mineral-thermobarometry techniques. These data feed into broader research on cratonic mantle structure and the conditions of diamond stability.