A conoscope is an optical accessory used in conjunction with a polariscope to produce convergent, polarised light through a gemstone, generating an interference figure that reveals the stone's optic character and, in many cases, its optic sign. Where the standard polariscope examines a gem in orthoscopic (parallel-ray) mode to determine whether it is singly or doubly refracting, the conoscope goes further: it compresses a wide cone of polarised light through the specimen so that rays travelling at many different angles are observed simultaneously, producing a characteristic pattern of coloured rings and dark brushes from which a trained gemmologist can determine whether the stone is uniaxial or biaxial, and whether its optic sign is positive or negative. This information is a significant aid in gemstone identification, since optic character is a fixed physical property of a mineral species.

Optical Principles

All transparent crystalline gemstones that are not cubic (isotropic) are anisotropic — they split incident light into two rays that travel at different velocities, a property called double refraction or birefringence. The geometry of this splitting is governed by the crystal system. Minerals belonging to the tetragonal, trigonal, and hexagonal systems have a single direction along which no double refraction occurs: the optic axis. These are uniaxial minerals. Minerals belonging to the orthorhombic, monoclinic, and triclinic systems have two such directions and are biaxial.

When convergent polarised light passes through an anisotropic gem, rays at different inclinations experience different degrees of retardation. Where the retardation equals a whole wavelength, destructive interference produces dark bands; where it equals a half-wavelength, constructive interference produces colour. The resulting pattern — the optic figure or interference figure — encodes the crystal's symmetry in a form directly visible through the eyepiece.

Instrument Design and Configuration

In its most common gemmological form, the conoscope is a two-component addition to a standard polariscope:

• Condensing lens — a high-power converging lens placed beneath the gem stage. This focuses the polarised light from the lower polariser into a cone of rays, each travelling at a slightly different angle through the stone. The numerical aperture of this lens determines the angular range of the figure and hence how much of the interference pattern is visible.
• Bertrand lens (or pinhole cap) — placed above the upper polariser (analyser), this lens re-focuses the back focal plane of the condensing system onto the observer's eye, making the interference figure visible. A simple pinhole cap can substitute for the Bertrand lens, though with some loss of brightness and clarity.

Some dedicated gemmological polariscopes incorporate a rotating stage, allowing the operator to orient the gem so that the optic axis (or one of the two optic axes in a biaxial stone) is aligned with the optical path — the ideal position for observing a centred figure. A conoscope sphere, a small glass or synthetic sphere used as a coupling medium between the condensing lens and the flat-bottomed gem, is sometimes employed to improve light entry and reduce surface reflections, particularly for small or irregularly shaped specimens.

Reading the Interference Figure

The two principal figure types correspond to the two optic classes:

• Uniaxial figure — when the optic axis is aligned with the microscope axis, the figure appears as a series of concentric coloured rings (isochromes) crossed by a dark cross (isogyres) whose arms remain fixed as the stage is rotated. The centre of the cross coincides with the optic axis. This pattern is diagnostic of uniaxial minerals such as corundum (ruby and sapphire), quartz, tourmaline, and zircon. If the optic axis is slightly off-centre, the cross shifts but retains its characteristic form.
• Biaxial figure — the figure is more complex. When viewed along the acute bisectrix (Bxa), two curved isogyres appear that separate into hyperbolic brushes as the stage is rotated, sweeping apart and returning in a characteristic figure-of-eight motion. The separation of the isogyres at 45° from extinction is related to the optic axial angle (2V). Minerals such as alexandrite (chrysoberyl), topaz, peridot, tanzanite, and spinel-group members (though spinel itself is isotropic and shows no figure) are biaxial.

Optic sign — whether a uniaxial mineral is optically positive or negative, or whether a biaxial mineral's acute bisectrix is the fast or slow ray — can be determined by introducing a sensitive tint plate (a first-order red plate or quarter-wave plate) into the optical path. The colour shifts in the quadrants of the figure indicate sign. Optic sign alone rarely identifies a gem species, but combined with refractive index, birefringence, and specific gravity, it substantially narrows the field of candidates.

Practical Considerations in Gemmological Use

Conoscopic examination is most reliable on faceted stones of at least 3–5 mm in diameter and reasonable transparency. Very small stones, heavily included specimens, or strongly coloured gems may yield poorly defined figures. The gem must be immersed or placed in contact with the condensing lens — dry contact often suffices for larger stones, while a drop of immersion liquid with a refractive index close to that of the gem can improve coupling and reduce stray reflections.

Orientation is critical. A uniaxial stone examined perpendicular to its optic axis will show a flash figure (a broad, diffuse cross that sweeps rapidly across the field on rotation) rather than the diagnostic centred cross, and may be mistaken for a biaxial figure by an inexperienced observer. Patience in rotating and tilting the specimen to find the centred figure is essential. In practice, gemmologists often examine a stone in several orientations before drawing a conclusion.

Mounted stones present an additional challenge: the setting may obstruct the condensing lens or prevent adequate rotation. In such cases, conoscopic examination may be inconclusive, and the gemmologist must rely on other tests.

Role in Gemstone Identification

The conoscope is particularly valuable in distinguishing species that share similar refractive indices or colours. A classic example is the separation of synthetic from natural corundum: both are uniaxial negative, but the conoscopic figure may reveal anomalous biaxial character in certain synthetic stones or in natural corundum subjected to heat treatment that has induced strain. Similarly, distinguishing blue topaz (biaxial) from aquamarine (uniaxial) is straightforward connoscopically even when their refractive indices overlap at the limits of a standard refractometer's range.

Assembled stones and doublets may show confusing or split figures if the two components have different optic characters. This can itself be a diagnostic clue to the composite nature of the stone.

In a fully equipped gemmological laboratory, conoscopic examination is one component of a systematic identification protocol alongside refractive index measurement, specific gravity determination, spectroscopic analysis, and, where warranted, advanced techniques such as EDXRF or Raman spectroscopy. The conoscope's value lies in its speed and non-destructive character: it requires no sample preparation and yields results in seconds once the operator is practised.

Historical Context

Conoscopic observation originated in mineralogy and crystallography during the nineteenth century, where the polarising microscope — equipped with a Bertrand lens, named after the French mineralogist Émile Bertrand — became a standard tool for identifying rock-forming minerals in thin section. The technique was adapted for gemmological use as polariscopes became standard laboratory instruments in the early twentieth century. GIA's foundational gemmological curricula have included conoscopic technique since the mid-twentieth century, and the method is described in detail in the GIA's Gem Identification Lab Manual and in the course materials for the Graduate Gemologist programme.

Further Reading

• GIA Gems & Gemology — gia.edu
• GIA Gem Identification resources — gia.edu
• International Gem Society: Using the Polariscope — gemsociety.org