The Eickhorst refractometer is a gemmological instrument manufactured by Eickhorst GmbH, a Hamburg-based German maker of precision optical equipment. Designed to measure the refractive index (RI) of polished gemstones, it belongs to the class of critical-angle refractometers that form the backbone of practical gemmological identification. Alongside the Rayner and GIA-pattern instruments, the Eickhorst refractometer has established itself as one of the principal tools used in European trade laboratories, valued for its robust mechanical construction, consistent calibration, and optical clarity.

Principle of Operation

All critical-angle refractometers, including the Eickhorst, operate on the principle that light passing from a denser medium into a less dense medium is refracted at the interface. The instrument incorporates a hemicylindrical glass prism of high refractive index — typically around 1.81 to 1.89, depending on the glass formulation — against whose flat face the gemstone is placed using a contact liquid of known, intermediate refractive index (commonly methylene iodide-based fluids or proprietary equivalents with an RI near 1.79 to 1.81). Monochromatic light, standardised at the sodium D-line wavelength of 589 nm, illuminates the assembly. The boundary between light and shadow that forms on the instrument's calibrated scale corresponds directly to the critical angle, from which the refractive index of the stone is read.

For anisotropic gemstones — those belonging to all crystal systems except cubic — the instrument reveals two shadow edges when the stone is rotated, corresponding to the two principal refractive indices (or, in the case of uniaxial stones, the ordinary and extraordinary rays). The difference between these readings gives the birefringence, itself a diagnostically useful value.

Construction and Design

Eickhorst instruments are characterised by a metal body of substantial weight and finish consistent with German precision-instrument manufacture. The eyepiece incorporates a magnifying lens that renders the shadow edge and the engraved RI scale simultaneously in focus, allowing readings to be taken to approximately ±0.002 RI units under good conditions. The hemicylindrical prism is recessed into the instrument's upper surface, protecting it from accidental contact while still permitting easy placement of faceted stones. A polarising filter, rotatable by the operator, is typically included to allow separation of the two shadow edges in birefringent stones and to assist in distinguishing uniaxial from biaxial optical character.

The instrument is designed for use with an external monochromatic light source directed through a window at one end of the body, though many practitioners use a fibre-optic or LED sodium-equivalent source in modern laboratory settings.

Range and Limitations

The practical measurement range of the Eickhorst refractometer, in common with most instruments of this type, extends from approximately 1.40 to 1.81 RI. Stones with refractive indices above this ceiling — including demantoid garnet (approximately 1.88), sphalerite (approximately 2.37), and diamond (2.417) — fall outside the readable range and yield only a dark field with no discernible shadow edge, a result described in gemmological practice as an over-the-limit reading. Such a result is itself diagnostically informative, as it immediately restricts the identification to a relatively small group of high-index species.

Accurate readings require a flat, well-polished facet of sufficient size to make adequate contact with the prism surface. Curved surfaces, heavily abraded facets, or very small stones may produce indistinct or unreadable shadow edges. The contact liquid must be applied sparingly to avoid contamination of the prism surface, which requires periodic cleaning with a suitable solvent.

Place in Gemmological Practice

Refractive index measurement remains one of the most reliable first-line tests in gemstone identification, and the refractometer — in whichever manufacturer's form — is accordingly a standard fixture in any equipped gemmological laboratory. The Eickhorst instrument is particularly prevalent in German-speaking countries and across continental European trade laboratories, reflecting both the manufacturer's regional base and a broader tradition of German optical instrument manufacture. It is approved for use in examinations conducted by the Deutsche Gemmologische Gesellschaft (DGemG) and is familiar to graduates of European gemmological programmes.

In practical use, the Eickhorst refractometer is employed alongside the polariscope, spectroscope, and Chelsea colour filter as part of a systematic identification sequence. RI values obtained with the instrument are compared against established reference ranges for gemstone species — ranges codified in standard references such as Robert Webster's Gems and the GIA's instructional materials — to narrow identification to species, and in some cases to variety or geographic origin when combined with other observations.

Further Reading

• GIA — The Refractometer in Gemology (Gems & Gemology)
• International Gem Society — Using a Refractometer