Octahedron Inclusion — Eight-Faced Crystals Trapped in a Host Gem
Octahedron Inclusion — Eight-Faced Crystals Trapped in a Host Gem
A characteristic crystal-inclusion morphology, especially diagnostic for diamond-in-diamond and spinel-group inclusions
An octahedron inclusion is a crystal inclusion exhibiting octahedral morphology — eight triangular faces arranged as two square pyramids joined base-to-base — characteristic of minerals that crystallise in the cubic system. In gemmology, octahedral inclusions are a useful diagnostic feature: when seen inside a host stone, they indicate that the included crystal grew with cubic symmetry and they often allow specific identification of the included mineral. In diamond, octahedral inclusions are most often smaller diamond crystals (the protogenetic diamond-in-diamond) or spinel-group minerals; in other host gems, octahedral inclusions of magnetite, pyrite, or various spinel-group species are diagnostic of natural origin and growth environment.
Crystallographic basis
The octahedron is one of the simple forms of the cubic crystal system, characterised by eight equivalent triangular faces meeting at six vertices and twelve edges. Minerals crystallising in cubic point groups can grow as octahedra under appropriate conditions. Common gem-relevant minerals with octahedral habit include diamond, spinel, magnetite, fluorite, pyrite, and certain garnets. The presence of an octahedral crystal inside a host stone indicates that the included mineral grew with cubic symmetry, immediately narrowing the identification.
Diamond-in-diamond inclusions
The most celebrated octahedral inclusion in gemmology is the diamond-in-diamond — a smaller diamond crystal trapped inside a host diamond during growth. These inclusions are particularly significant because they are protogenetic — formed before the host crystal began to grow — and their presence provides direct evidence of the growth environment. They demonstrate that the host diamond grew in a region where multiple diamond crystals were nucleating and growing simultaneously, with the smaller crystal becoming engulfed by the larger as both grew.
Diamond-in-diamond inclusions are valued by collectors and researchers studying diamond formation. The morphology of the included crystal, combined with isotopic and trace-element analysis where laboratory access permits, can reveal the conditions under which the host diamond formed. For the trade, diamond-in-diamond inclusions are diagnostic of natural origin — synthetic diamonds rarely contain other diamond crystals as inclusions because of the very different growth environment in HPHT and CVD synthesis.
Spinel and other octahedral inclusions in diamond
Octahedral spinel-group inclusions in diamond — including chromite, magnesiochromite, and various Mg-Al-Cr spinels — are common and diagnostic of mantle origin. The composition of these inclusions provides information about the depth and chemistry of the mantle source region from which the diamond originated. Peridotitic suite (P-type) inclusions in diamonds reflect formation in mantle peridotite, while eclogitic suite (E-type) inclusions reflect formation in mantle eclogite; the inclusion suite is a primary tool in classifying diamond formation environments.
For the laboratory and trade, octahedral spinel inclusions in diamond confirm natural origin. Synthetic diamonds — both HPHT and CVD — rarely contain crystalline mineral inclusions of mantle-derived origin; their typical inclusions are metallic flux residues (in HPHT) or graphitic and pinpoint inclusions (in CVD), with morphologies and compositions distinguishable from natural mantle inclusions.
Octahedral inclusions in other gems
In coloured stones, octahedral inclusions appear as magnetite, pyrite, spinel, or chromite crystals, depending on the host species and its formation environment. Magnetite inclusions are common in metamorphic and hydrothermal gem hosts including some sapphires and rubies. Pyrite octahedra appear in emerald — particularly in Colombian Trapiche and certain Brazilian and Zambian material — where they are diagnostic of the formation environment.
The recognition of octahedral inclusions during routine examination provides immediate clues about the host's geological context, often supporting attribution to a specific source or formation type. In origin determination by major laboratories, inclusion morphology is one of several lines of evidence that, combined with trace-element chemistry and other data, supports confident attribution.