The Nicol dichroscope is the modern variant of the dichroscope, the handheld optical instrument used to display the two pleochroic colours of a doubly refractive gemstone side by side for direct comparison. The name honours William Nicol (1768 to 1851), the Scottish geologist and physicist whose 1828 invention of the polarising calcite prism — the original Nicol prism — provided the polarisation technology on which the modern dichroscope depends. Nicol's original calcite prism construction has been substantially superseded in contemporary instruments by polarising film (Polaroid) and by alternative prism designs, but the name Nicol dichroscope persists as a generic designation for any modern handheld dichroscope using polarisation rather than the older calcite-cleavage approach.

The pleochroism phenomenon

Pleochroism is the property by which a doubly refractive gemstone shows different colours when viewed along different crystallographic axes. The phenomenon arises from the differential absorption of light vibrating along different crystal directions, with each direction having its own characteristic absorption spectrum. In a uniaxial crystal — hexagonal, trigonal, or tetragonal — the pleochroism is dichroic, with two distinct colours appearing along the two principal vibration directions (the ordinary and extraordinary rays). In a biaxial crystal — orthorhombic, monoclinic, or triclinic — the pleochroism is trichroic, with three distinct colours appearing along the three principal axes.

The two principal pleochroic colours are visible side-by-side in a dichroscope when the instrument is held to the stone and rotated to find the orientation of maximum colour contrast. The resulting visual difference can be diagnostic for the species, can confirm the identity of stones with characteristic strong pleochroism (tanzanite, iolite, andalusite, kunzite, tourmaline), and can support the orientation of rough material before cutting to maximise the face-up colour of the finished stone.

The Nicol prism and its modern descendants

The original Nicol prism, as devised by William Nicol in 1828, consists of a calcite rhombohedral crystal cleaved at a specific angle, cut into two halves, and recemented with Canada balsam. The construction exploits the birefringence of calcite to separate the two polarisation components of the incident light, with one component refracted into the prism and emerging as polarised output and the other component reflected at the cement layer and absorbed at the prism walls. The prism produces highly polarised output — better than 99.9 per cent for well-made specimens — and was the principal polarising element in scientific instruments for most of the nineteenth and twentieth centuries.

Modern polarisation technology has substantially superseded the calcite Nicol prism in routine instruments. Polarising film (the Polaroid material developed by Edwin Land from 1929 onwards) provides moderately polarised output (typically 99 per cent for high-quality polarising film) at much lower cost and with much easier mechanical handling, and is the standard polariser in contemporary handheld dichroscopes. For instruments requiring higher polarisation purity or specific optical characteristics, alternative prism designs — Glan-Foucault, Glan-Thompson, Glan-Taylor, Wollaston — have largely replaced the original Nicol design in scientific applications.

The name Nicol dichroscope has nonetheless persisted as a generic designation for any modern handheld dichroscope using polarisation, regardless of whether the polarising element is a calcite prism (now uncommon), polarising film (most common), or one of the modern alternative prism designs (occasional). The use of the name is essentially honorific rather than technically specific.

Construction and use

A modern Nicol dichroscope is a small handheld optical instrument, typically a few centimetres long, consisting of a rectangular aperture at one end through which the gemstone is viewed, a polarising element behind the aperture, and an eyepiece at the other end. The polarising element is oriented to display the two principal vibration directions of the stone side-by-side as separate windows in the field of view. Some designs use a single polariser with a calcite splitter to produce the two-window image; others use crossed polarisers; the optical principle is similar in either case.

To use the instrument, the gemmologist holds the stone close to the aperture against a strong light source, brings the eye close to the eyepiece, and rotates either the stone or the dichroscope to find the orientation of maximum colour contrast between the two windows. Multiple rotations from different stone orientations may be required to identify the principal pleochroic colours, particularly in trichroic biaxial stones where three different colour pairs may be visible from different orientations.

Best results are achieved with strong directional light — daylight from a window or a high-quality desk lamp — and with a stone clean of surface contamination. The instrument is held still close to the eye while the stone is rotated; the orientation that produces the strongest colour contrast is the diagnostic orientation, and the two colours visible in the dichroscope at that orientation are the principal pleochroic colours of the species.

Diagnostic applications

The dichroscope is one of the standard handheld gemmological tools, alongside the loupe, refractometer, polariscope, and spectroscope. Its principal diagnostic applications include the confirmation of stone identity through characteristic pleochroism (tanzanite is famously trichroic with violet, blue, and yellow-brown pleochroism that immediately confirms the species), the distinction of doubly refractive stones from singly refractive simulants (cubic zirconia is singly refractive and shows no pleochroism, distinguishing it immediately from doubly refractive natural stones such as zircon or topaz), and the orientation of rough material before cutting.

Strong-pleochroism species — tanzanite, iolite (cordierite), andalusite, kunzite, alexandrite chrysoberyl, blue tourmaline, ruby, and certain sapphires — are the principal candidates for dichroscope examination. Weakly pleochroic species (most beryl, most quartz, most garnet) show subtle dichroism that requires careful examination and good lighting to detect. Singly refractive species (diamond, garnet, fluorite, glass) show no pleochroism and the dichroscope confirms that absence as a diagnostic feature.

Limitations

The dichroscope's principal limitation is that it requires the operator to find the orientation of maximum colour contrast, and on faceted stones this can require multiple rotations through different orientations. Stones with very small face-up dimensions — small mêlée — can be difficult to examine effectively. Stones in mountings, particularly closed-back settings, can be difficult or impossible to examine through the dichroscope.

The instrument provides qualitative rather than quantitative information about pleochroism. The two colours are described in subjective terms (violet versus blue-grey; yellow-green versus brown), and quantitative measurement of the pleochroism — for research or detailed characterisation purposes — requires a polarising microscope rather than a handheld dichroscope.

In the trade

For working gemmologists, dealers, and trade gemmologists, the dichroscope is part of the basic toolkit and is used routinely in stone identification. The instrument is inexpensive, the technique is straightforward to learn, and the diagnostic value is high for the strong-pleochroism species. The standard gemmological education curricula — GIA, FGA, IGA — include dichroscope use as a fundamental skill, and proficiency develops with experience across the range of species commonly encountered in the trade.

Further reading

• GIA Gems & Gemology — pleochroism and dichroscope references
• International Gem Society — gemmological optics references
• ICA — coloured-stone identification reference
• Lotus Gemology — handheld gemmology techniques