A gypsum plate — also called a first-order red plate or sensitive tint plate — is a thin optical accessory inserted into a polariscope to determine the optic sign of birefringent gemstones. It consists of a carefully cleaved or ground slice of gypsum (CaSO₄·2H₂O) oriented and mounted so that it introduces a fixed optical path difference, or retardation, of approximately 550 nm. This value corresponds precisely to first-order red on the Michel-Lévy interference colour chart, producing a characteristic magenta or sensitive-tint background when the plate is placed between crossed polarising filters.

Optical Principle

When plane-polarised light passes through a birefringent material, it is resolved into two rays — the fast ray and the slow ray — that travel at different velocities and therefore accumulate a phase difference. The gypsum plate introduces its own fixed retardation along a known slow-ray direction, conventionally marked on the plate mount. When the plate is inserted into the optical path of a polariscope already displaying an interference figure, the retardations of the stone and the plate either add or subtract depending on their relative orientations.

If the slow ray of the gemstone under examination is oriented parallel to the slow ray of the gypsum plate, the retardations add, shifting the interference colours toward higher orders — from magenta toward blue. If the slow rays are perpendicular (i.e., the fast ray of the stone aligns with the slow ray of the plate), the retardations subtract, shifting the colours toward lower orders — from magenta toward yellow or orange. This colour shift, observed in the quadrants of a conoscopic interference figure, reveals whether the stone is optically positive or negative.

Use in Optic-Sign Determination

The gypsum plate is employed after a clear interference figure has been centred in the polariscope eyepiece using convergent light. For a uniaxial stone, a centred optic-axis figure shows concentric isochromatic rings crossed by a dark cross (isogyres). Upon insertion of the gypsum plate:

• In the quadrants where the stone's slow ray parallels the plate's slow ray, the colour rises toward blue — these quadrants are said to show addition.
• In the quadrants where the stone's fast ray parallels the plate's slow ray, the colour falls toward yellow — these quadrants show subtraction.

The position of the blue (addition) quadrants relative to the plate's slow-ray direction determines the optic sign: if addition occurs in the northeast and southwest quadrants (with the plate's slow ray oriented northeast–southwest), the stone is optically positive; if addition occurs in the northwest and southeast quadrants, the stone is optically negative. For biaxial stones, the same logic applies to the acute bisectrix figure, with the curvature and separation of the isogyres providing additional information about the optic axial angle (2V).

Construction and Specifications

The plate is typically mounted in a thin brass or plastic slide sized to fit the accessory slot of a standard polariscope or petrographic microscope. The gypsum crystal is oriented so that its slow vibration direction — corresponding to the lower refractive index direction in gypsum, which is the b-crystallographic axis — runs at a defined angle, usually 45° to the polariser direction or along a marked axis on the mount. The 550 nm retardation is sensitive to small changes in thickness, which is why the term sensitive tint is apt: at this precise retardation, even minor additions or subtractions of path difference produce vivid, easily read colour shifts. Retardation plates producing other fixed values (such as a quarter-wave plate at 137 nm) exist for specialised work, but the first-order red plate remains the standard for routine gemmological optic-sign testing.

Practical Considerations

Obtaining a clean, centred interference figure is a prerequisite; without it, the colour shifts introduced by the gypsum plate cannot be interpreted reliably. Small or heavily included stones, as well as those with very low birefringence, may yield indistinct figures that make optic-sign determination difficult regardless of the accessory used. The plate itself should be handled carefully: gypsum is a soft mineral (Mohs hardness 2), and the mounted slice, though protected by glass cover slips in quality instruments, can be damaged by impact or moisture. A cracked or delaminated plate will produce uneven retardation and unreliable results.

In routine gemmological practice, the gypsum plate is most frequently used to distinguish between uniaxial positive stones (such as quartz and zircon) and uniaxial negative stones (such as tourmaline and calcite), and to confirm the biaxial character and optic sign of species such as topaz, chrysoberyl, and the feldspar group. It is a modest but indispensable tool in any well-equipped gemmological laboratory.

Further Reading

• GIA — The Polariscope in Gemology
• International Gem Society — Using the Polariscope