Apatite Inclusion
Apatite Inclusion
Hexagonal calcium phosphate crystals as gemmological fingerprints in corundum and beyond
An apatite inclusion is a solid crystal of the mineral apatite — calcium phosphate with the general formula Ca5(PO4)3(F,Cl,OH) — enclosed within a host gemstone during its formation. Apatite itself belongs to the hexagonal crystal system and typically presents as short prismatic to acicular crystals, commonly colourless or white in the context of gem inclusions, though pale yellow and greenish tints are occasionally observed. Because apatite is an extraordinarily widespread accessory mineral in igneous, metamorphic, and sedimentary environments, its occurrence as an inclusion is correspondingly broad; however, it carries particular diagnostic weight in corundum from specific localities, most notably marble-hosted rubies from Mogok, Burma, and metamorphic sapphires from Sri Lanka.
Mineralogy and Morphology
Apatite crystallises in the hexagonal system (space group P63/m) and has a hardness of 5 on the Mohs scale — considerably softer than its corundum host (hardness 9). This disparity means that apatite inclusions are mechanically stable within corundum but can, under certain cutting and polishing conditions, create stress fractures or feathers radiating from the crystal boundary. The refractive index of apatite (approximately 1.63–1.64) differs sufficiently from that of corundum (1.76–1.77) to render the crystals visible as bright, high-relief objects under the microscope, often displaying a characteristic hexagonal outline in cross-section or a stubby barrel form in three-quarter view. Under fibre-optic or darkfield illumination, the crystals may show a glassy to resinous lustre distinct from the surrounding corundum.
Occurrence in Mogok Ruby
The marble-hosted rubies of Mogok, in the Mandalay Region of Myanmar, are perhaps the most celebrated context in which apatite inclusions acquire diagnostic significance. The Mogok metamorphic belt formed through regional metamorphism of carbonate sediments, and the resulting gem-bearing marbles carry a characteristic mineral assemblage that includes calcite, dolomite, phlogopite, graphite, and apatite. Within Mogok rubies, apatite crystals frequently appear in association with calcite and dolomite crystals, sometimes clustered together in groups that directly reflect the composition of the enclosing marble. This suite of carbonate-associated inclusions is a primary criterion used by major gemmological laboratories — including the Gübelin Gem Lab, SSEF, and GIA — when assigning a marble-type, Mogok-consistent origin to a ruby. The presence of apatite alongside calcite, in particular, is considered strong supporting evidence for a marble host-rock origin, distinguishing these stones from basalt-hosted rubies (such as those from Mong Hsu, Vietnam, or Thailand) in which apatite inclusions are either absent or occur without the carbonate mineral suite.
Occurrence in Sri Lankan Sapphire
Sri Lankan (Ceylon) sapphires and other corundum varieties from the island's gem gravels — derived from metamorphic rocks of the Highland Complex — also commonly host apatite inclusions. Here, apatite may appear as isolated prismatic crystals or as part of a broader metamorphic mineral assemblage that can include zircon, rutile, and feldspar. The apatite crystals in Sri Lankan corundum are sometimes larger and more euhedral than those observed in Mogok material, and their association with zircon halos (pleochroic haloes caused by radiation damage from zircon's uranium and thorium content) is a well-documented feature of Ceylon-origin stones. Gemmologists examining a sapphire with apatite, zircon, and rutile silk in combination have strong grounds for considering a Sri Lankan provenance, though origin determination always requires a holistic assessment of multiple inclusion types, trace-element chemistry, and spectroscopic data.
Diagnostic Value and Origin Determination
The value of apatite inclusions in origin determination lies not in the mineral itself — which is too ubiquitous to be locality-specific in isolation — but in its paragenetic context: the accompanying mineral suite, the morphology of the crystals, and their spatial relationship within the host stone. Gemmological laboratories employ a combination of standard gemological microscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for trace-element fingerprinting, and ultraviolet-visible spectroscopy to build a complete origin profile. Apatite inclusions contribute to this profile as one component of a broader mineralogical argument.
It is worth noting that apatite inclusions are not confined to corundum. They have been documented in garnets, tourmalines, feldspars, and various other gem species. In each case, the same interpretive principle applies: the inclusion's significance is contextual, read against the backdrop of the host mineral's known geological environments.
Practical Considerations for the Trade
For dealers and appraisers, the presence of apatite inclusions in a ruby or sapphire is generally neutral with respect to clarity grading unless the crystals are large, prominently positioned, or associated with fractures that compromise transparency. Small, well-enclosed apatite crystals in a Mogok ruby may, paradoxically, add value by supporting a laboratory report confirming marble-type origin — one of the most commercially significant origin designations in the coloured-stone market. Conversely, the same inclusions in a stone submitted for origin determination without supporting trace-element data may be insufficient on their own to confirm provenance.