Reflection Inclusion
Reflection Inclusion
The mirror-image artefact that produces ghost features in faceted stones
A reflection inclusion is an optical artefact in which a real internal feature appears duplicated because a polished facet behaves as an internal mirror, returning a secondary image of the feature elsewhere in the stone. The duplicate is not an additional inclusion; it is the same crystal, fluid pocket, or feather seen twice. Recognising the artefact is part of routine clarity grading, because counting the reflection as a separate feature would over-state the inclusion content of the stone and mis-grade it.
How the artefact forms
When an inclusion sits near one pavilion or crown facet of a faceted gem, light scattering from the inclusion can travel across the interior, meet an opposite facet at greater than the critical angle for total internal reflection, and return to the eye as a coherent image visible from the table. The reflected image typically appears displaced from the original and often inverted, mirrored across the plane of the reflecting facet. In symmetrical brilliant cuts where pavilion mains and breaks are oriented at predictable angles, a single inclusion can generate two, three, or even more reflected images depending on facet geometry and viewing angle.
The phenomenon is most pronounced in stones with high refractive index, where the critical angle for total internal reflection is smaller and a larger fraction of light striking the pavilion facets is returned rather than transmitted. Diamond, with a critical angle of about 24.4 degrees, supports particularly efficient internal reflection and consequently shows reflection inclusions readily. Sapphire, with a critical angle around 34 degrees, shows them less prolifically but still routinely. Lower-RI stones such as quartz produce reflection inclusions only weakly and within narrower viewing windows.
Identification
The diagnostic test is rotation. The original inclusion remains fixed within the stone as the gem is rotated under the loupe or microscope; a reflection moves, distorts, or disappears as the geometry between the feature, the reflecting facet, and the viewer changes. Experienced graders trace the reflection back to its source by tilting the stone systematically and watching the secondary image collapse into the primary one as the reflecting facet falls out of alignment. A useful technique is to dim the surrounding light and use a single point source: the reflection brightens and dims as the stone is rocked, while the original inclusion remains constant.
Reflection inclusions can also be confirmed by oblique fibre-optic illumination from the side, which often picks out the original feature sharply while leaving the reflections faint or absent. Microscopists working at gem laboratories develop a strong intuition for the geometry of the cut and can predict where reflections will occur before they hunt for them.
In the trade
Reflection inclusions are documented extensively in the Gübelin Photoatlas of Inclusions and are part of every coloured-stone grader's training. They are most prominent in cleanly cut, well-polished stones with high refractive index — diamond, sapphire, demantoid garnet, zircon — where pavilion facets behave as efficient internal mirrors. An accurate inclusion plot ignores the reflections and records only the primary features, which is one reason laboratory clarity reports are more reliable than casual loupe assessments by inexperienced examiners.
For appraisers and dealers, the practical consequence is that rapid loupe inspection in the trade should always include rotation. A stone that appears at first glance to contain three crystal inclusions may in fact contain a single small crystal whose reflection produces two ghost images. Mistaking reflections for separate features can move a stone unnecessarily down the clarity scale.