Optic sign is a property of doubly refractive (anisotropic) gemstones that indicates the relative velocity of the two rays produced by double refraction. In uniaxial stones, if the extraordinary refractive index (n epsilon) is greater than the ordinary refractive index (n omega), the stone is optically positive; if lower, it is optically negative. In biaxial stones, the classification is based on whether the acute bisectrix of the two optic axes coincides with the gamma (slowest) vibration direction (positive) or the alpha (fastest) direction (negative). Optic sign is determined by observing interference figures with an accessory plate, and the result aids in distinguishing species with similar refractive indices.

Determination by accessory plate

The standard method for determining optic sign uses an accessory plate inserted into the optical path of a polarising microscope or conoscope while observing the interference figure. The most common accessory plates are the quartz wedge (which produces a continuous variation in retardation) and the first-order red plate (which produces a fixed retardation of approximately 550 nanometres, equivalent to the first-order red interference colour).

For a uniaxial figure, the gemmologist observes how the colour bands shift when the accessory plate is inserted. If the colour bands in the upper-right and lower-left quadrants of the cross shift toward higher-order interference colours (blue), and the bands in the upper-left and lower-right quadrants shift toward lower-order colours (yellow), the crystal is optically positive. The opposite shift indicates optically negative. The mnemonic 'addition versus subtraction' captures the underlying physics: adding the accessory-plate retardation to the crystal's intrinsic retardation produces the colour shift.

For biaxial crystals, the technique is similar but applied to the bisectrix view of the figure rather than a single optic axis. The colour shifts in the isochromes around the melatope points indicate the optic sign in the same way.

Diagnostic value

Optic sign provides additional identification information beyond optic character (uniaxial or biaxial) and refractive index. Where two species have similar refractive indices and similar optic characters, the optic sign can resolve the identification. For example, sapphire is uniaxial negative, while quartz is uniaxial positive — the difference in optic sign distinguishes them where refractive index alone might not be diagnostic.

For biaxial species, optic sign combines with the 2V angle to characterise the optical character more completely. Tanzanite is biaxial positive with a small 2V; topaz is biaxial positive with a medium 2V; chrysoberyl is biaxial positive with a large 2V. The combination of biaxial-positive classification, 2V value, and refractive index range together provides reliable identification.

Common gem species and optic signs

Common uniaxial-negative gem species include corundum (sapphire and ruby), tourmaline, beryl (emerald and aquamarine), calcite, and apatite. Common uniaxial-positive species include zircon, quartz, and rutile. Common biaxial-positive species include topaz, chrysoberyl, and most varieties of tanzanite. Common biaxial-negative species include peridot (depending on iron content) and many feldspars.

The optic sign is a stable property of the species rather than of individual stones, although chemical zoning or substantial trace-element variation can occasionally produce variation in optic sign within a single specimen. For routine identification, the published reference values for optic sign apply to the bulk of stones in each species.

Practical considerations

Optic-sign determination requires the stone to be oriented appropriately for the observation. For loose stones, the gemmologist may need to rotate the stone in immersion oil until an optic-axis figure or bisectrix figure appears; for set stones, the orientation is constrained by the setting and may not always permit clean optic-sign determination. The technique requires modest practice to perform reliably; gemmologists working through their training typically encounter optic-sign determination as part of standard laboratory exercises.

For field-buying and rapid identification, optic-sign determination is generally not undertaken; the more elaborate setup required for accessory-plate observation is reserved for laboratory work. Field identification proceeds on the basis of refractive index, specific gravity, dichroism, and visual inspection, with optic-sign reserved for confirmation in difficult cases.

In the trade

For working gemmologists, optic-sign is one element of the broader optical-mineralogical characterisation of a species, used for confirmation and for resolving ambiguous identifications. Standard gemmological references provide the optic-sign of all common gem species, and laboratory identifications include the optic-sign as part of the documented characterisation.

See also uniaxial, biaxial, optic axis, optic character, optic angle 2V, conoscope, interference figure, and optical mineralogy for related entries.

Further reading

• GIA Gems & Gemology — optical mineralogy and identification
• GIA — gemmological practice reference
• International Gem Society — optic sign and identification