The calcite dichroscope is a hand-held optical instrument used in gemmology to observe pleochroism — the property by which certain doubly refractive gemstones transmit different colours along different crystallographic axes. The device employs a cleavage rhomb of optically clear calcite, historically known as Iceland spar, to split a beam of light entering a gemstone into two orthogonally polarised rays. These two rays are presented simultaneously to the observer's eye as two adjacent colour windows, allowing direct comparison of the pleochroic colours without any need for rotating polarising filters. The calcite dichroscope remains the original and, in many practitioners' estimation, the most optically elegant solution to this diagnostic task.

Historical Background

The instrument was introduced by the Austrian mineralogist Wilhelm von Haidinger in 1847. Haidinger recognised that the extreme birefringence of calcite — with refractive indices of approximately 1.486 and 1.658, giving a birefringence of 0.172 — made a simple rhomb of the mineral an effective natural beam-splitter requiring no additional optical components. Iceland spar, sourced principally from large transparent cleavage masses found in Iceland and later from deposits in Mexico and elsewhere, had long been known for this splitting effect; Haidinger's contribution was to formalise it into a compact diagnostic tool for mineralogy and, subsequently, gemmology.

Optical Principle

Calcite is a uniaxial negative mineral belonging to the trigonal system. When unpolarised light enters the rhomb, it is divided into an ordinary ray (o-ray) and an extraordinary ray (e-ray), each vibrating in planes perpendicular to one another. Because a pleochroic gemstone absorbs these two polarisation directions selectively, each ray emerges carrying a different colour or depth of colour. The calcite rhomb separates these rays sufficiently that, when viewed through the eyepiece aperture of the dichroscope, they appear as two distinct rectangular fields side by side. The observer rotates the stone — not the instrument — to explore different crystallographic directions, identifying the full range of pleochroic colours.

The instrument is effective only on doubly refractive (anisotropic) stones. Singly refractive (isotropic) materials such as spinel, garnet, and glass show identical colour in both windows regardless of orientation, which is itself diagnostically useful.

Construction

A standard calcite dichroscope consists of three principal components:

• A calcite rhomb of sufficient optical clarity and thickness to achieve adequate ray separation — typically a few centimetres in length.
• A cylindrical housing, usually of brass or anodised aluminium, with a small square or rectangular aperture at the stone end to admit a narrow beam of light.
• A lens or eyepiece at the observer's end to bring the two colour fields into sharp focus.

No polarising filters, batteries, or moving parts are required. The instrument functions entirely by the inherent optical properties of the calcite crystal, making it robust, maintenance-free, and indefinitely serviceable provided the rhomb remains unscratched and unfogged.

Use in Gemmological Practice

To use the calcite dichroscope, the gemmologist holds the instrument close to the eye, positions the stone against the aperture, and illuminates it from behind with a diffuse white light source — a fibre-optic lamp or a simple daylight-balanced bulb. The stone is rotated slowly while the two windows are observed. In strongly pleochroic stones, the colour difference between the two fields is immediately apparent; in weakly pleochroic stones, careful comparison is required.

Gemstones for which the calcite dichroscope is particularly diagnostic include:

• Tanzanite (trichroic: blue-violet, red-violet, and yellow-green in its three principal axes — the dichroscope reveals two of these at any given orientation).
• Iolite (cordierite): strongly trichroic, showing deep violet-blue, pale blue, and yellowish in different directions.
• Ruby and sapphire: ruby shows purplish-red versus orangey-red; blue sapphire shows blue versus greenish-blue.
• Alexandrite: trichroic, with colours varying from red to orange to green depending on direction.
• Andalusite: strongly trichroic in green, red, and yellow-brown.

Calcite Dichroscope versus Polarising-Filter Dichroscope

A second design of dichroscope, introduced later, substitutes two pieces of polarising film oriented at 90° to one another for the calcite rhomb. Both types accomplish the same fundamental task, but practitioners note differences in performance. The calcite instrument generally produces brighter, more saturated colour fields because it relies on physical beam separation rather than selective absorption by polarising film; there is no light lost to the filter material itself. The polarising-filter dichroscope, however, is less sensitive to the precise quality and clarity of its optical element, and replacement filters are straightforward to source. For teaching and field use, both remain current; many professional gemmologists keep both types to hand.

Limitations

The calcite dichroscope cannot be used on isotropic stones, opaque stones, or stones so heavily included that light cannot be transmitted. Very weakly pleochroic materials — some beryls, for instance — may show differences too subtle to assess reliably without supplementary spectroscopic or polariscopic examination. The instrument also does not distinguish between the three axes of a trichroic stone in a single observation; multiple orientations must be examined to document the full pleochroic scheme.

Further Reading

• GIA Gems & Gemology — notes on dichroscope use
• International Gem Society — Pleochroism in Gemstones