The bull's-eye optic figure is an interference figure observed through a polariscope when a uniaxial gemstone is oriented with its optic axis parallel to the line of sight and examined between crossed polarising filters. The figure presents as a series of concentric alternating dark and light rings — the isochromes — centred on a sharp dark cross formed by two intersecting bands known as isogyres. Its resemblance to a target or bull's-eye gives the figure its common name. For gemmologists, it is among the most immediately diagnostic observations available without recourse to a refractometer, serving as reliable confirmation that a stone belongs to the uniaxial optical class.

Optical Basis

Minerals crystallising in the tetragonal, trigonal, or hexagonal systems possess a single optic axis — the crystallographic direction along which light travels without experiencing double refraction (birefringence). When polarised light enters the stone along this axis, rays travelling at progressively greater angles to it are split into ordinary and extraordinary components that accumulate phase differences proportional to the path length and the birefringence of the material. Under crossed polarisers, these phase differences produce the characteristic pattern of concentric extinction and retardation rings. The dark cross of isogyres marks the directions of vibration parallel to the polariser and analyser; because these directions experience no net retardation relative to each other, they remain extinguished regardless of stage rotation, giving the cross its fixed, stable appearance — a key distinction from the biaxial interference figure, in which the isogyres curve and separate upon rotation.

Gemstones That Commonly Display the Figure

Any uniaxial species can, in principle, yield a bull's-eye figure when suitably oriented. In routine gemmological practice, the figure is most frequently encountered in:

• Corundum (ruby and sapphire) — trigonal; a clean bull's-eye confirms uniaxial character and is consistent with natural or synthetic corundum alike.
• Quartz — trigonal; the figure is readily obtained in well-cut or rough material oriented along the c-axis, and is a standard teaching example in polariscope instruction.
• Tourmaline — trigonal; the figure is obtainable but may be less distinct owing to the typically strong pleochroism and darker body colour of many tourmaline varieties.
• Idocrase (vesuvianite) and apatite — tetragonal and hexagonal respectively; both uniaxial and capable of displaying the figure under favourable orientation.

Biaxial species — including chrysoberyl, topaz, peridot, and the feldspar group — will never produce a true bull's-eye. Attempting to obtain the figure and finding instead the curved, separating isogyres of a biaxial figure, or a diffuse flash figure, is itself diagnostic information.

Practical Technique

Obtaining a well-defined bull's-eye requires careful orientation of the stone. The gemstone is placed on the polariscope stage between crossed polarisers with a condensing lens (or a loupe held close above the stone) to converge the light cone sufficiently. The stone is then tilted or rotated until the concentric ring pattern centred on the dark cross comes into view. In faceted gems, the table facet oriented perpendicular to the optic axis — as in many round brilliant-cut sapphires or quartz — often provides the most accessible viewing direction. Thicker stones and those with higher birefringence tend to show more rings; very thin or weakly birefringent specimens may show only the central cross with few or no surrounding rings.

The clarity and symmetry of the figure are also affected by strain. A stone under significant internal stress — whether from growth, cutting, or thermal shock — may show a distorted or fragmented cross, a useful secondary observation in its own right.

Gemmological Significance

The bull's-eye figure contributes to gem identification in several practical ways. It confirms uniaxial optical character, immediately excluding all biaxial species from consideration. When a stone suspected to be sapphire or ruby yields a clean bull's-eye, this is consistent with corundum; failure to produce any interference figure — as would occur with glass or a cubic synthetic such as cubic zirconia or synthetic garnet — points toward an isotropic imitation. Some synthetic corundum grown by the Verneuil (flame-fusion) process exhibits anomalous double refraction due to strain, and the resulting distorted or irregular figure can alert the gemmologist to investigate further with other instruments. The polariscope observation is non-destructive, requires no immersion liquid, and takes only seconds, making the bull's-eye figure a valuable first-line diagnostic tool.

Further Reading

• GIA — Using the Polariscope (Gems & Gemology)
• International Gem Society — Polariscope Use in Gem Identification