Polariscope Figure
Polariscope Figure
The pattern of isogyres and isochromes that visualises a stone's optic-class geometry under convergent polarised light
The polariscope figure is the pattern of dark and light bands — isogyres and isochromes — observed when a gemstone is viewed in a polariscope or conoscope under convergent polarised light. The figure visualises the stone's optic-axis geometry: uniaxial stones show a centred or off-centre dark cross; biaxial stones show two curved isogyre brushes. The figure rotates and changes shape as the stone is rotated, and the rotational behaviour itself carries diagnostic information.
Isogyres and isochromes
Isogyres are the dark bands in the figure — locations where the optical axis of the stone is parallel to one of the polarising filter axes, so the stone produces no birefringence at that location and the analyser blocks the light fully. Isochromes are the coloured rings or bands surrounding the isogyres — locations where the birefringence has produced a path-length difference between the two refracted rays equal to a specific fraction of a visible-light wavelength, and the resulting interference produces a particular colour. The colour of an isochrome depends on the stone's birefringence and the path length through the stone at that field position.
Uniaxial figures
For a uniaxial stone with the optic axis along the viewing direction, the figure is a centred dark cross — four isogyre arms meeting at the centre — surrounded by concentric coloured isochrome rings. Rotation of the stone leaves the cross unchanged; the figure does not break apart. For a uniaxial stone with the optic axis tilted away from the viewing direction, the cross is off-centre and the isochromes are eccentric. Strong rotation may bring the cross out of the field of view entirely.
Biaxial figures
For a biaxial stone, the figure shows two curved isogyre brushes — hyperbolic curves — that may converge into a centred figure or remain separated, depending on the optic-angle 2V. As the stone is rotated, the brushes move apart and back together, with the maximum separation reached at 45-degree rotational positions. The 2V angle can be estimated from the maximum separation, with smaller separations indicating smaller 2V values. The figure direction also distinguishes biaxial-positive from biaxial-negative stones when combined with a sensitive-tint accessory.
Practical observation
Figures are most readily observed with the stone immersed in a refractive-index-matched fluid (glycerine, methylene iodide), with the conoscope lens or sphere lens fitted, and with the analyser orientation adjusted for sharpest contrast. The stone must be oriented to bring an optic axis as close to the viewing direction as possible; tilting and translating the stone in the immersion cell is part of the technique. For faceted stones, the table-down or pavilion-up orientation typically gives the cleanest figure, since the table provides a flat optical-quality surface at the entry to the stone.
Faint figures are diagnostic. Some stones — particularly those with low birefringence (apatite, beryl) or very small size (melee, accent stones) — produce figures that are visible only briefly or in particular orientations. The trained operator learns to recognise these faint figures and to interpret them in the context of the broader identification. Strong figures are obvious; faint figures require patience and good light.
In the trade
For Skyjems and other working dealers, polariscope figure observation is part of the standard mid-level identification routine — applied after the loupe and before refractive-index measurement on stones whose species is not immediate from inspection. The figure quickly distinguishes the major optic classes, and the rotational behaviour adds confidence to the determination. Combined with refractive-index, specific-gravity, and dichroscope observations, the figure is one of the inputs to the identification chain that supports working purchase decisions.