A conoscope cap is a small, snap-on accessory — typically moulded in opaque plastic — that fits over the eyepiece of a standard gemological refractometer, converting it into a rudimentary conoscope. By introducing convergent polarised light through a doubly refractive gemstone, the device produces optic figures (interference figures) that allow the gemmologist to determine a stone's optic character, optic sign, and, in some cases, to support identification or distinguish natural material from synthetic counterparts. Despite its modest cost and unassuming appearance, the conoscope cap extends the diagnostic reach of the refractometer considerably beyond simple refractive-index measurement.

How It Works

In normal refractometer use, light travels through the stone in a single, essentially parallel path. The conoscope cap replaces the standard eyepiece lens arrangement with a short-focal-length condensing lens system that causes light to pass through the gemstone at a range of angles simultaneously — that is, as a convergent beam. When this convergent light exits the stone and is viewed through a polarising filter (usually incorporated into the cap itself, crossed against the refractometer's existing polariser), the anisotropic optical properties of the crystal produce a characteristic interference pattern visible at the centre of the field of view.

The resulting figure takes one of two principal forms:

• Uniaxial figure: A centred or off-centred dark cross (isogyres) superimposed on concentric coloured rings (isochromes), characteristic of crystals belonging to the trigonal, tetragonal, or hexagonal systems — including corundum, quartz, and tourmaline.
• Biaxial figure: A more complex pattern in which the isogyres curve or separate into two hyperbolic arcs, characteristic of orthorhombic, monoclinic, and triclinic crystals — including chrysoberyl, topaz, and feldspars.

Isotropic stones (cubic system and amorphous materials such as glass) produce no figure at all, which is itself diagnostic.

Optic Sign Determination

Beyond simply classifying a stone as uniaxial or biaxial, an experienced gemmologist can use the conoscope figure to assess optic sign — whether the stone is optically positive or negative. In a uniaxial figure, optic sign is read by observing the sequence of colour addition or subtraction in the quadrants of the cross when a sensitive-tint plate (first-order red plate) is inserted; in a biaxial figure, the curvature and separation of the isogyres relative to the melatope positions provide the same information. Optic sign is a useful confirmatory property: corundum, for instance, is uniaxial negative, while zircon is uniaxial positive — a distinction that can assist in separating these visually similar stones when refractive indices alone are ambiguous due to surface condition or cut.

Practical Considerations

The conoscope cap is most effective on stones with a flat, polished surface of reasonable size — a table facet or a large flat face — pressed firmly against the refractometer's hemicylinder. Heavily included or very small stones yield degraded or unreadable figures. The technique also requires the stone to be oriented so that an optic axis (or the acute bisectrix in biaxial stones) is approximately perpendicular to the observation direction; off-axis orientations produce off-centred or partial figures that demand experience to interpret correctly.

Immersion in a contact liquid of appropriate refractive index is not required for the conoscope cap in the same way as for a polariscope-based conoscopic setup, since the refractometer's own contact liquid serves to couple the stone to the hemicylinder. However, the field of view is considerably smaller and the figure less well-developed than that obtainable with a dedicated polariscope fitted with a proper conoscopic lens — a point worth noting when results are equivocal.

Place in the Gemmological Toolkit

The conoscope cap occupies a practical middle ground: it adds conoscopic capability to an instrument the gemmologist already owns, without requiring a separate polariscope and conoscopic lens. For routine laboratory work, a dedicated polariscope with a proper Bertrand lens or conoscopic attachment will generally yield cleaner, larger, and more easily interpreted figures. Nevertheless, the conoscope cap remains a standard accessory supplied or available for most major refractometer models — including those produced by Rayner and Krüss — and is a legitimate first step in optic-character determination when a polariscope is not at hand.