Optic Figure — The Interference Pattern Under Convergent Light
Optic Figure — The Interference Pattern Under Convergent Light
The interference pattern observed when viewing a gemstone down an optic axis under crossed polars and convergent light
The optic figure is the interference pattern observed when a gemstone is viewed down an optic axis under crossed polars and convergent light, typically using a conoscope or polarising microscope with a Bertrand lens. Uniaxial stones display a centred black cross (the uniaxial cross) with concentric coloured rings; biaxial stones show curved dark bands called isogyres that open and close as the stone is rotated. The optic figure reveals the stone's optic character (uniaxial or biaxial) and, with appropriate accessory plate use, the optic sign (positive or negative). Together with the 2V angle for biaxial crystals, this information is fundamental to optical-mineralogical species identification.
The instrument setup
Observation of the optic figure requires three optical components in addition to the standard microscope: crossed polars (a polariser below the stage and an analyser above the objective, with their vibration directions at right angles), a substage condenser to provide convergent light, and a Bertrand lens (or equivalent telescope eyepiece) that magnifies the image at the back focal plane of the objective rather than the standard image plane. The Bertrand lens is the key feature: without it, the conoscope view of the back focal plane is too small to interpret usefully.
The conoscope is the dedicated instrument for this purpose, but most polarising microscopes include the necessary components and a Bertrand lens that can be inserted into the optical path. For gemmological use, the conoscope or properly equipped polarising microscope is essential equipment in any laboratory undertaking serious identification work.
The uniaxial figure
For a uniaxial crystal viewed along its optic axis, the conoscope figure shows a black cross at the centre of the field with concentric coloured rings extending outward. The cross arms (the isogyres) correspond to the directions in which the analyser blocks all light. The coloured rings (the isochromes) correspond to lines of equal interference colour, with the colour sequence following the standard interference colour series (grey, white, yellow, red, blue, etc.) outward from the centre.
As the stage rotates, the cross remains stationary and the rings remain concentric — this is the diagnostic feature of the uniaxial figure that distinguishes it from biaxial. If the optic axis is not perfectly centred in the field of view, the cross appears off-centre but its arms still remain stationary as the stage rotates.
To determine optic sign, an accessory plate (typically a quartz wedge or a first-order red plate) is inserted into the optical path. The colour shift in the rings, particularly along the cross arms, indicates whether the crystal is optically positive or negative. The technique requires practice but provides reliable optic-sign determination once familiar.
The biaxial figure
For a biaxial crystal viewed close to one of its two optic axes, the conoscope figure shows a single curved isogyre that rotates as the stage rotates. Viewed at the bisectrix between the two optic axes, the figure shows two isogyres that open and close — appearing to flex apart and back together — as the stage rotates through 90 degrees. The maximum separation of the isogyres at the bisectrix view corresponds to the 2V angle of the crystal.
Coloured rings (isochromes) are also present in biaxial figures, with a more complex geometry than the concentric rings of the uniaxial figure. The colours follow the same interference-colour sequence but are arranged around the two melatope points (the centres of the optic-axis interference patterns) rather than around a single centre.
For optic-sign determination in biaxial crystals, the same accessory-plate technique is used, with the colour shifts in the isochromes indicating positive or negative character. Combined with the 2V measurement, the biaxial figure provides complete optical-mineralogical characterisation.
Identification use
The optic figure is a fundamental identification tool for biaxial gem species and an important confirmation tool for uniaxial species. The combination of optic character (uniaxial or biaxial), optic sign (positive or negative), and 2V angle for biaxial crystals narrows most identifications to a small set of candidate species, which can then be confirmed with refractive-index measurement and other tests.
The technique requires the stone to be oriented appropriately for observation. For loose stones, the gemmologist may need to rotate the stone in immersion oil on the stage until an optic-axis figure appears; for set stones, the orientation is constrained by the setting. Some stones present interference figures that are difficult to interpret because of zoning, internal stress, or imperfect orientation; these require additional patience and sometimes additional confirmatory tests.
In the trade
For working gemmologists, the optic figure is one of the standard observations in the identification protocol, alongside refractive index, specific gravity, dichroism, and microscopic inspection. The conoscope or polarising microscope is found in any rigorous gemmological laboratory, and the technique is taught as part of standard gemmological education. For field work, the more elaborate conoscope setup is generally not portable, and identification proceeds on the basis of more easily transportable tests.
See also conoscope, optic axis, optic character, optic sign, optic angle 2V, uniaxial, biaxial, and interference figure for related entries.