The polariscope-conoscope combination is the integrated bench gemmological instrument that combines orthoscopic polariscope and conoscope functions in a single unit, allowing the operator to switch between parallel-light and convergent-light modes without changing instruments. In polariscope mode, the instrument determines optic character — isotropic, uniaxial, or biaxial — through the standard crossed-polar rotation test. In conoscope mode, an additional converging lens is brought into the light path to produce interference figures, distinguishing uniaxial from biaxial within the anisotropic class and providing the optic-sign determination.

Construction

A typical combo unit consists of a base housing a diffuse white light source, a lower polariser, a rotating stone stage, an upper analyser, and a switchable conoscope lens — a sphere lens or Bertrand lens — that swings or slides into the optical path on demand. Better units include an immersion cell that fills with a refractive-index-matched fluid (typically a glycerine or methylene-iodide solution) to improve figure visibility, an adjustable analyser orientation, and intensity control on the light source. A small viewing magnifier above the analyser brings the figure into eye focus.

Manufacturers including Kassoy, GemOro, Eickhorst, and Krüss produce combo units across a price range from teaching-grade portable models through laboratory-grade benchtop instruments. The optical-quality differences appear most clearly in conoscope mode, where high-quality lenses produce sharper figures on small or pale stones than budget instruments can match.

Standard procedure

The operator places the stone on the stage and runs the polariscope test first — rotation through 360 degrees with observation of brightness behaviour. If the stone is anisotropic, the operator brings the conoscope lens into the light path and adjusts the stone orientation to bring an optic axis as close to the viewing direction as possible. Immersion in the refractive-index-matched fluid sharpens the figure. The operator observes the resulting interference pattern: a centred or off-centre cross indicates uniaxial; two curved isogyre brushes indicate biaxial.

For optic-sign determination, the operator inserts a quartz wedge or a 530 nm sensitive-tint accessory into the light path and observes the colour shift in the figure. The pattern of colour movement determines whether the stone is uniaxial positive, uniaxial negative, biaxial positive, or biaxial negative. Combined with the basic polariscope result, refractive-index measurement, and dichroscope observation, the optic-class and optic-sign information narrows species candidates substantially.

Position in the bench setup

Combo units occupy the middle ground between the basic polariscope (cheaper, less informative) and the dedicated polarising microscope (more expensive, more informative). For most working dealer benches, the combo is the right tool — sufficient optical-class information for routine identification work without the cost and complexity of a research-grade petrographic microscope. Major laboratories use combo units for rapid screening and reserve polarising-microscope work for cases requiring the higher resolution.

In the trade

For Skyjems and other working dealers, the combo unit is the standard bench instrument behind the loupe. The two-position rotation test plus the conoscope figure determination handles a high proportion of routine identification, particularly for the species pairs where optic class is diagnostic — quartz vs. topaz, ruby vs. spinel, peridot vs. tourmaline, and the various beryl, tourmaline, and corundum simulants. The instrument earns its bench space through the speed and informativeness it provides.

Further reading

• GIA Gems & Gemology — bench instrumentation articles
• International Gem Society — polariscope and conoscope reference
• American Gem Trade Association — gemmological testing materials