The Eickhorst dichroscope is a calcite-prism dichroscope manufactured by Eickhorst GmbH, a Hamburg-based maker of gemmological instruments with a long-standing reputation in the European trade. Like all calcite dichroscopes, it exploits the strong birefringence of Iceland spar (optical-grade calcite) to split an incoming beam of light into two orthogonally polarised rays, which the observer sees simultaneously as two adjacent colour windows. In doubly refractive, pleochroic gemstones — those whose crystal structure causes them to absorb different wavelengths along different optical axes — the two windows display distinct colours or colour intensities, providing immediate diagnostic information without the need for electrical power or complex calibration.

Optical Principle

A calcite rhomb placed in the optical path of the instrument separates incident light into an ordinary ray and an extraordinary ray, each vibrating in planes perpendicular to one another. When the instrument is held up to a strongly illuminated stone and rotated through 90 degrees, the colours seen in the two windows exchange positions; this interchange confirms genuine pleochroism rather than an artefact of surface colour zoning. The Eickhorst instrument presents these two windows side by side through a small eyepiece, allowing direct colour comparison under identical lighting conditions. The quality of the calcite rhomb is the single most important determinant of instrument performance: optical-grade material with minimal inclusions and well-polished faces produces the cleanest colour separation.

Construction and Design

Eickhorst dichroscopes are characterised by robust metal-tube construction, a format standard among professional gemmological instruments intended for bench use. The calcite rhomb is housed in a protective mount within the tube, shielded from humidity and mechanical shock — conditions relevant in workshop environments. The eyepiece end is fitted with a small aperture that frames the twin colour windows, and the opposite end accepts light transmitted through or reflected from the stone under examination. The instrument requires no battery or light source of its own; a fibre-optic lamp, a daylight-equivalent LED, or even a well-positioned window provides adequate illumination.

Diagnostic Applications

The dichroscope is most informative when used on strongly pleochroic species. Among the stones most frequently examined with the Eickhorst instrument in European trade laboratories are:

• Tanzanite (trichroic): displays blue, violet, and burgundy-red along its three optical directions — one of the most vivid trichroic responses observable with a dichroscope, and a rapid means of separating tanzanite from blue glass or synthetic spinel.
• Iolite (Cordierite): shows a pronounced shift between blue-violet and near-colourless or pale yellow, earning it the historical trade name dichroite.
• Tourmaline: typically shows strong dichroism between a deep body colour and a lighter or differently hued tone, with the degree varying by variety.
• Sapphire and ruby (corundum): both are uniaxial and show dichroism between their ordinary and extraordinary rays; in natural blue sapphire the two windows typically show blue and blue-green or greenish-blue, a response that differs from some synthetic corundum grown by certain methods, though definitive separation of natural from synthetic requires additional testing.
• Alexandrite and other chrysoberyls: alexandrite's trichroism — red, orange, and green — is readily confirmed with a dichroscope before more elaborate colour-change testing.

Isotropic stones (cubic-system gems such as diamond, spinel, and garnet, as well as glass) show identical colour in both windows regardless of orientation, which is itself a useful negative result.

Place in the Gemmological Toolkit

The dichroscope occupies a specific and irreplaceable niche in the gemmologist's instrument set. It is inexpensive relative to a refractometer or spectroscope, requires no consumables, and delivers an instant qualitative result that can guide subsequent, more time-consuming testing. The Eickhorst instrument, alongside comparable calcite dichroscopes from other European and American makers, is listed as a standard piece of bench equipment in gemmological curricula including those of the Gemmological Association of Great Britain (Gem-A) and the Deutsche Gemmologische Gesellschaft (DGemG). Its continued presence on jewellers' benches across Europe reflects the enduring utility of a simple optical principle executed in well-made hardware.