Doubly Refractive Doubling of Inclusions
Doubly Refractive Doubling of Inclusions
How birefringence creates ghost images within a gemstone
Doubly refractive doubling of inclusions — sometimes called doubled inclusions — is an optical phenomenon in which an inclusion enclosed within a birefringent gemstone appears not as a single image but as two overlapping or slightly displaced images when viewed through the host material. The effect is a direct consequence of the host stone's birefringence: as light passes through an anisotropic crystal, it is split into two rays travelling at different velocities and along slightly different paths, each forming its own image of the same internal feature. The result, visible under a loupe or microscope, is a characteristic ghosting or doubling that serves as a reliable diagnostic tool in practical gemmology.
The Optical Mechanism
In an isotropic material — glass, garnet, spinel — light travels at a single velocity regardless of direction, and any inclusion presents one clean image. In an anisotropic (doubly refractive) material, the crystal's internal symmetry causes incident light to resolve into an ordinary ray and an extraordinary ray, each refracted at a different angle. When both rays pass through or around an inclusion, they each carry an image of that feature to the observer's eye, separated by a distance proportional to the stone's birefringence value and the depth of the inclusion within the host. The greater the birefringence, the more pronounced the lateral displacement between the two images.
Species in Which the Effect Is Most Pronounced
Zircon is the gemstone most immediately associated with this phenomenon in trade practice. High-type zircon carries a birefringence of approximately 0.059, one of the highest values among commonly faceted gems. Needle-like inclusions, crystal fragments, and even facet edges viewed through the pavilion appear strikingly doubled, and this doubling is routinely cited as a primary field identification feature for zircon. Metamict zircon — in which radiation damage has partially destroyed the crystal lattice — shows reduced or absent doubling, providing an additional layer of diagnostic information.
Calcite exhibits extreme birefringence of approximately 0.172, the highest of any common mineral encountered in gemmology. The classic demonstration of the principle uses a cleavage rhomb of Iceland spar (optical-quality calcite), through which a dot of ink resolves into two clearly separated images visible to the naked eye. Calcite occurs as an inclusion mineral in a variety of host gems, and where it is present, its own internal doubling — or the doubling of features within it — can assist identification.
Other species where the effect is diagnostically useful include peridot (birefringence approximately 0.036), zoisite (including tanzanite), topaz, and tourmaline. In lower-birefringence species such as quartz (approximately 0.009) or corundum (approximately 0.008), the doubling is present in principle but rarely perceptible without magnification and careful orientation.
Practical Diagnostic Application
When examining an unknown stone under a gemological microscope or 10× loupe, the gemmologist rotates the specimen while observing internal features. In a doubly refractive host, inclusions will appear to double as the stone is turned, with the degree of separation shifting with orientation and reaching a maximum when the observation direction is perpendicular to the optic axis. In a singly refractive stone, no such doubling occurs regardless of orientation. This test is particularly valuable when a refractometer reading is unavailable — for instance, with mounted stones or very small specimens — and provides rapid confirmation of the host material's optical character.
It is worth noting that doubling of inclusions should not be confused with the apparent doubling of facet edges seen through the table of a high-birefringence stone such as zircon; both effects share the same cause, but the former involves internal features while the latter involves the stone's own facets viewed in reflection or transmission.