Dichroscope
Dichroscope
The calcite instrument for observing pleochroism in gemstones
The dichroscope is a compact, hand-held optical instrument used in gemmology to observe and compare the pleochroic colours displayed by anisotropic gemstones. At its core lies a rhomb of optically clear calcite — historically known as Iceland spar — which exploits the mineral's strong birefringence to split a single beam of transmitted light into two orthogonally polarised rays. These two rays are presented simultaneously as adjacent colour windows in the eyepiece, allowing the gemmologist to compare, side by side, the colours vibrating along different crystallographic directions. The dichroscope is a standard diagnostic tool in gem identification curricula worldwide, including those of the Gemological Institute of America (GIA) and the International Gem Society (IGS).
Optical Principle
Calcite is strongly birefringent: when unpolarised light enters the rhomb, it is resolved into an ordinary ray (o-ray) and an extraordinary ray (e-ray), each vibrating at 90° to the other. Because a pleochroic gemstone absorbs different wavelengths of light along different vibrational directions, the two rays emerging from the calcite carry distinct colour information. The eyepiece presents these as two small rectangular fields of colour — typically separated by a thin dark line — that can be compared directly. When the stone is rotated, the colours in the two windows change, revealing the full range of pleochroic hues.
Dichroism and Trichroism
The number of distinct pleochroic colours a gemstone can display depends on its crystal system:
- Isotropic stones (cubic system, such as spinel, garnet, and diamond) show no pleochroism. Both windows appear identical in colour regardless of orientation.
- Uniaxial stones (tetragonal, hexagonal, and trigonal systems) exhibit dichroism — a maximum of two distinct colours, corresponding to the ordinary and extraordinary rays. Classic examples include ruby and sapphire (corundum), where the difference between the o- and e-ray colours can be diagnostically significant.
- Biaxial stones (orthorhombic, monoclinic, and triclinic systems) can exhibit trichroism — up to three distinct colours along three principal optical directions (alpha, beta, and gamma). Tanzanite is a celebrated example, displaying blue, violet, and burgundy depending on viewing direction.
Because the dichroscope presents only two rays at any one time, observing all three colours in a trichroic stone requires rotating the specimen through multiple orientations.
Construction and Use
The instrument consists of a short tube, typically 8–12 centimetres in length, with a calcite rhomb mounted centrally, a small aperture at the stone end to admit a narrow beam of light, and a lens at the eyepiece end to focus the two colour windows. A light source — ideally a strong, diffuse white light such as a fibre-optic lamp or a daylight-balanced LED — is directed through the stone from behind, and the gemmologist views through the eyepiece while slowly rotating the stone. The stone should be examined through several faces and orientations to ensure all pleochroic colours are recorded.
Correct technique requires that the stone be held close to the aperture and that the light source be sufficiently bright; pale or heavily included stones may yield inconclusive results. Cabochons and heavily faceted stones can be more challenging to examine than well-polished flat surfaces, though experienced practitioners can work around these limitations.
Diagnostic Applications
The dichroscope is particularly valuable in several identification scenarios:
- Species separation: Red stones that appear visually similar — ruby, red spinel, red garnet, and red glass — can be quickly differentiated. Ruby (corundum) shows strong dichroism (purplish-red and orangey-red), red spinel shows none (cubic), and red garnet shows none (cubic). A single glance through the dichroscope can eliminate entire species.
- Orientation of rough: In cutting, knowledge of pleochroic directions allows the lapidary to orient the stone so that the most desirable colour faces up through the table facet. This is especially important for alexandrite, tanzanite, and strongly pleochroic tourmalines.
- Synthetic versus natural: While the dichroscope alone cannot confirm natural origin, anomalous or absent pleochroism in a stone that should be pleochroic can raise suspicion of glass, synthetic cubic zirconia, or other simulants.
- Identification of unknown stones: The presence, strength, and specific hues of pleochroism contribute meaningfully to a gemmological identification sequence alongside refractive index, specific gravity, and spectroscopic data.
Limitations
The dichroscope is a qualitative rather than quantitative instrument. It indicates the presence and approximate character of pleochroism but does not measure it numerically. Very pale stones, heavily included specimens, or stones with strong body colour may be difficult to assess. Additionally, the instrument requires transmitted light, making opaque or near-opaque stones impractical to examine. For definitive identification, the dichroscope is used in conjunction with a refractometer, spectroscope, and, where necessary, advanced laboratory techniques.