Polariscope
Polariscope
The crossed-polar instrument that separates isotropic from anisotropic stones in seconds
The polariscope is the gemmological instrument consisting of two polarising filters arranged with their transmission axes perpendicular — a lower polariser and an upper analyser, separated by a stage on which the stone is placed. Crossed polars produce a dark field when no stone is present; rotating an unidentified stone in the field reveals its optic character — isotropic or anisotropic — through the pattern of brightness changes observed during rotation. The instrument is a daily-use bench tool in every working gem laboratory and a foundational component of the standard gemmological setup.
Construction
A standard polariscope consists of a base housing a diffuse white light source, a lower polariser sheet, a stone stage (typically a glass disc that rotates), and an upper analyser sheet mounted at fixed 90-degree orientation to the polariser. Better instruments include a rotating analyser, a removable conoscope lens (for interference-figure work), an immersion cell for pavilion-up examination, and adjustable light source intensity. The optical path is short and the instrument fits on a bench corner; portable hand-held models exist for field use.
Standard procedure
The stone is cleaned and placed pavilion-down on the stage with the analyser already in place. The light source is turned on and the stone is rotated through 360 degrees while the operator observes brightness changes. An isotropic stone — cubic-system minerals, glass, most amorphous organics — remains dark or shows uniform low brightness throughout rotation. An anisotropic stone — every other crystal system — alternates between bright and dark four times per rotation, with the bright positions corresponding to azimuths where the stone's optical axes lie at 45 degrees to the polariser axes.
Variations on the basic test detect special cases. Anomalous double refraction — strain birefringence in nominally isotropic stones — produces brightness patterns that follow strain rather than crystallographic axes; garnet often shows ADR. Aggregate stones — chalcedony, jadeite, lapis — show four-position brightness behaviour identical to single-crystal anisotropic stones because the aggregate is made of randomly oriented anisotropic crystallites. The interpretation rule is that aggregate ADR-like response is distinguishable from true anisotropic response under closer examination, and a polariscope determination should be confirmed against refractive index, specific gravity, and microscopic observation.
Interference figures
With a conoscope lens (sometimes called a polariscope cap) inserted above the stone, the polariscope becomes a conoscope. The lens converges the light passing through the stone, illuminating it from a range of angles simultaneously, and the resulting interference pattern visible through the analyser distinguishes uniaxial stones (centred cross or off-centre cross) from biaxial stones (curved isogyre brushes). The conoscope determination supplements the basic polariscope optic character with optic-class information that the simple crossed-polar test does not provide.
Limitations
The polariscope cannot determine species directly. It separates the broad optical classes — isotropic, uniaxial, biaxial — and the result narrows the species candidates rather than identifying them. The follow-up tests of refractive index and specific gravity, combined with microscopic observation and pleochroism (via dichroscope), are needed for species identification. The polariscope also cannot detect treatment, origin, or synthesis status; it is a first-pass triage tool, not an identification instrument.
In the trade
For Skyjems and other working dealers, the polariscope is the first instrument applied to any unidentified parcel after the loupe. The two-instrument routine — loupe plus polariscope — handles the majority of routine purchase decisions on commercial material, and the few stones that do not yield to it are flagged for closer laboratory work. Bench polariscopes from manufacturers including GemOro, Kassoy, Eickhorst, and Krüss are the standard choices, with the variations among instruments principally in light source quality and conoscope feature inclusion rather than in core function.