Epigenetic Inclusion
Epigenetic Inclusion
Post-formation features that entered the host crystal through fractures and structural pathways
An epigenetic inclusion is any inclusion that formed or was introduced into a host crystal after that crystal had completed its primary growth. The term derives from the Greek epi (upon or after) and genesis (origin), and it stands as one of three temporal categories used in gemmological inclusion science — the others being protogenetic (pre-existing material engulfed during growth) and syngenetic (material trapped contemporaneously with crystal growth). Epigenetic inclusions are of considerable practical importance: they record the post-depositional history of a stone, can influence clarity grading, and frequently serve as diagnostic evidence in origin determination and treatment detection.
How Epigenetic Inclusions Form
Once a crystal has grown, it remains susceptible to physical and chemical change. Tectonic stress, hydrothermal activity, metamorphic overprinting, or simple mechanical shock can open fractures within the stone. Fluids — aqueous solutions, silica-rich hydrothermal brines, or carbonic fluids — may then infiltrate these fractures. If the fracture subsequently heals through a process of dissolution and re-precipitation of the host mineral, the fluid becomes trapped as a thin, planar array of microscopic cavities. The result is a healed fracture, or, when the fluid-filled cavities are sufficiently numerous and arranged in a lacy, fingerprint-like pattern, a fingerprint inclusion.
In other cases, secondary minerals precipitate within open fractures rather than the fracture healing cleanly. Iron oxides, chlorite, calcite, and various phyllosilicates are commonly encountered secondary mineral deposits of this kind. These infiltrated minerals are texturally distinct from syngenetic mineral inclusions: they follow fracture planes, often display dendritic or coating habits, and may cross-cut primary growth features such as colour zoning or earlier syngenetic inclusions.
Principal Types
- Healed fractures: Planar arrays of minute fluid inclusions marking the former path of a fracture that has re-sealed. Under magnification they appear as irregular, often curved planes of tiny negative crystals or two-phase (liquid–vapour) cavities. Common in corundum, quartz, and topaz.
- Fingerprint inclusions: A subset of healed fractures in which the fluid-inclusion array takes on a rounded, lacy, or whorled pattern closely resembling a human fingerprint. Particularly well documented in sapphire and ruby, where they are among the most frequently observed epigenetic features.
- Secondary mineral deposits: Minerals such as iron oxides (goethite, limonite), chlorite, or calcite that have infiltrated fractures after crystal formation. In emerald, for instance, secondary calcite or iron oxides along fractures are common and can assist in distinguishing natural stones from synthetic ones.
- Partially healed fractures (feathers): Fractures in an intermediate stage of healing, in which fluid inclusions occupy only portions of the fracture plane. These are of particular commercial relevance because they can affect durability and are central to the assessment of fracture-filling treatments.
Diagnostic Significance
Epigenetic inclusions are indispensable tools for the gemmological laboratory. Because the fluids and secondary minerals that infiltrate a fracture reflect the geochemical environment of the deposit, their composition can contribute to geographic origin determination. In Burmese rubies, for example, the character and distribution of healed fractures and associated fluid inclusions differ measurably from those in Thai or Mozambican material. Gemmological laboratories including the Gübelin Gem Lab, Gübelin and Pala's Photoatlas of Inclusions in Gemstones, and the GIA's research programme have documented these distinctions in considerable detail.
Epigenetic fractures are also central to the detection of clarity treatments. Fracture filling — the injection of glass, resin, or oil into open fractures — exploits the same pathways that natural epigenetic processes use. A gemmologist examining a stone for treatment will assess whether fractures are naturally healed (and thus epigenetic in origin) or have been artificially filled. Flash effect, flow structures within the filler, and anomalous relief under the microscope all help distinguish treated fractures from naturally healed ones.
Relationship to the Three-Part Inclusion Classification
The tripartite system of protogenetic, syngenetic, and epigenetic inclusions was systematised and popularised largely through the work of Eduard Gübelin and John Koivula, whose multi-volume Photoatlas of Inclusions in Gemstones remains the standard reference in the field. The system provides a consistent vocabulary that allows gemmologists worldwide to communicate precisely about the temporal origin of any given inclusion, a prerequisite for rigorous origin and treatment analysis.