The Eickhorst polariscope is a precision optical instrument manufactured by the Hamburg-based firm Eickhorst GmbH, widely used in gemmological laboratories across Europe and beyond for determining whether a gemstone is singly refractive (isotropic) or doubly refractive (anisotropic). It belongs to the broader class of polariscopes — instruments employing two polarising filters oriented at 90 degrees to one another — and is regarded as one of the more robustly constructed commercial instruments of its type, valued for the clarity of its optics and the durability of its mechanical components.

Optical Principle

The polariscope operates on a straightforward but diagnostically powerful principle. Light from a diffuse source passes first through a lower polarising filter (the polariser), which restricts it to a single plane of vibration. The gemstone under examination is placed on a rotating stage between this lower filter and a second filter above (the analyser), oriented with its transmission axis perpendicular to that of the polariser — the so-called crossed-polars configuration. In this arrangement, no light reaches the observer's eye unless the gemstone itself alters the plane of polarisation of the transmitted beam.

Doubly refractive (anisotropic) stones — including corundum, beryl, tourmaline, and the great majority of natural gem species — split incident light into two rays vibrating in different planes and travelling at different velocities. As such a stone is rotated through 360 degrees under crossed polars, it passes through four positions of extinction (darkness) and four positions of maximum brightness, producing the characteristic alternating light-and-dark pattern that confirms double refraction. Singly refractive (isotropic) stones — diamond, spinel, garnet, and glass — do not split the light beam and therefore remain uniformly dark throughout rotation, a condition known as extinction.

Construction and Design

Eickhorst instruments are manufactured to the standards expected of professional laboratory equipment. The polarising filters are of high quality, providing a deep, clean extinction field that makes subtle optical effects easier to read. The rotating stone platform is typically fitted with a gem clip or a small hemispherical depression to hold the stone securely during rotation. A diffuse transmitted-light source — traditionally a frosted-glass disc over an incandescent or LED lamp — provides even, glare-free illumination. The overall construction is compact enough for bench use yet sufficiently substantial to withstand the daily demands of a working gemmological laboratory.

Diagnostic Applications

The polariscope's primary utility lies in a small number of well-defined diagnostic tasks:

• Distinguishing isotropic from anisotropic stones. This is the instrument's foundational application. A stone that remains dark throughout a full rotation under crossed polars is confirmed as singly refractive, immediately excluding the majority of natural gem species and narrowing identification considerably.
• Separating diamond and spinel from synthetic moissanite. Synthetic moissanite (SiC), which closely resembles diamond in brilliance and thermal conductivity, is doubly refractive and will show the characteristic blink under crossed polars; diamond, being isotropic, will not. This single test provides rapid preliminary discrimination before more specialised instruments are employed.
• Detecting anomalous double refraction (strain birefringence). Isotropic materials subjected to internal stress — including glass, synthetic cubic zirconia, and occasionally natural garnets — may display irregular, patchy, or "tabby" extinction rather than uniform darkness. This anomalous extinction is a useful indicator of strain, poor crystal growth, or the presence of glass in a composite or imitation stone.
• Confirming optical character in ambiguous cases. When a refractometer reading is inconclusive — for instance, when a stone's refractive index falls beyond the instrument's range — the polariscope can confirm whether the material is singly or doubly refractive, guiding subsequent testing.

Limitations

The polariscope cannot, by itself, identify a gemstone species; it establishes optical character, not identity. Heavily included or very dark stones may be difficult to read clearly. Certain garnets, particularly those under stress, can show anomalous extinction that superficially mimics double refraction, requiring the gemmologist to interpret the pattern carefully rather than mechanically. The instrument is also ineffective for opaque materials, which do not transmit the polarised beam.

Place in the Laboratory

In European gemmological practice, the polariscope — alongside the refractometer, loupe, and spectroscope — is considered standard bench equipment. The Eickhorst model in particular has long been a fixture in German-speaking countries and in laboratories that favour German optical manufacture. It is recommended or listed as standard equipment in the curricula of the Deutsche Gemmologische Gesellschaft (DGemG) and is consistent with the instrument types described in gemmological training programmes affiliated with the Gemmological Association of Great Britain (Gem-A). For the working gemmologist, it remains one of the most rapid and unambiguous preliminary tests available — a matter of seconds' rotation can resolve questions that might otherwise require more time-consuming procedures.