Diamond-in-Diamond Inclusion
Diamond-in-Diamond Inclusion
A diamond crystal enclosed within another diamond — one of the rarest and most scientifically revealing inclusion types in gemmology
A diamond-in-diamond inclusion is a discrete crystal of diamond entirely enclosed within a larger host diamond. It belongs to the broader category of crystal inclusions — solid mineral phases trapped during a gem's growth — but stands apart in that both host and guest share identical chemistry. This makes the diamond-in-diamond inclusion simultaneously a clarity characteristic of commercial relevance and a geological document of exceptional scientific value. Such inclusions are uncommon even by the standards of included diamonds, and their presence invites careful study of multi-stage crystallisation in the Earth's mantle.
Formation and Geological Significance
Diamond crystallises at depths typically between 150 and 200 kilometres in the lithospheric mantle, under pressures of roughly 4.5 to 6 gigapascals and temperatures exceeding 900 °C. A diamond-in-diamond inclusion arises when a pre-existing diamond crystal — itself fully formed during an earlier growth episode — is subsequently enveloped by a second, larger diamond growing around it. The two generations of diamond may be separated by a considerable interval of geological time, and the compositional or isotopic differences between them can, in principle, be measured by researchers using techniques such as secondary ion mass spectrometry (SIMS) or cathodoluminescence imaging.
Because diamond is chemically inert and mechanically robust, an included diamond crystal survives the encapsulation process without reacting with or dissolving into the host lattice. The boundary between the two crystals may be sharp and crystallographically coherent, or it may show strain features detectable under cross-polarised light. Studies published in Gems & Gemology and related literature have used such inclusions to reconstruct episodes of mantle metasomatism and to establish that some diamonds are composite objects recording more than one period of carbon precipitation from mantle fluids.
Morphology and Identification
Included diamond crystals typically express the natural growth habit of diamond: most commonly an octahedron, occasionally a dodecahedron or a rounded transitional form. Under magnification — standard 10× loupe or gemological microscope — they appear as sharply faceted, glassy, colourless to faintly tinted crystals with a high adamantine lustre that distinguishes them from other common inclusions such as olivine, garnet, or sulphide phases. Their refractive index is identical to that of the host, so they do not produce the same relief contrast that a mineral of different optical density would; instead, they are often revealed by their geometric outline, by subtle strain halos in the surrounding host lattice, or by surface features where the inclusion approaches the facet plane.
Raman spectroscopy provides definitive identification: the characteristic diamond peak at approximately 1332 cm⁻¹ confirms the inclusion's identity unambiguously, and any shift in that peak relative to the host value can indicate residual pressure — a phenomenon known as remnant pressure — that itself carries information about the depth of formation.
Clarity Grading Implications
In the GIA clarity grading system, an enclosed diamond crystal is plotted and described as a crystal inclusion. Its impact on the clarity grade depends on its size, position, and visibility under the standard 10× magnification. A very small, centrally located included diamond may contribute only minimally to a grade, while a larger or more prominently positioned crystal could influence the grade meaningfully. Should the included crystal intersect a polished surface — as occasionally occurs when a cutter works close to an existing inclusion — it is reclassified as a knot, a surface-reaching crystal that may affect polish quality and durability assessment in that localised area. The GIA Gem Encyclopedia and grading documentation address knots as a distinct clarity characteristic for this reason.
In the Trade
Diamond-in-diamond inclusions are rarely highlighted in commercial descriptions, where crystal inclusions of any kind are generally regarded as detractors from clarity. However, among collectors of natural-phenomenon gemstones and within the scientific community, a diamond containing a well-formed, identifiable included diamond crystal carries a certain distinction. Laboratory reports from GIA and other major grading laboratories will note the inclusion type; a report comment identifying the inclusion as diamond can, in informed hands, add a layer of narrative interest to an otherwise modestly graded stone. The inclusion does not enhance value in conventional market terms, but it does authenticate the stone's entirely natural origin and speaks to the complexity of processes operating deep within the Earth.