Refractometer
Refractometer
The workhorse instrument of gem identification
The refractometer is the optical instrument that measures the refractive index of faceted gemstones using the critical-angle method. It is the workhorse of routine gem identification: every coloured-stone laboratory operates one, every gemmology programme teaches its use, and the great majority of identifications across the trade rest on its readings. The instrument has been in essentially its modern form since the early twentieth century, and despite the rise of Raman spectroscopy and energy-dispersive X-ray fluorescence as supplementary tools, the refractometer remains the first instrument the gemmologist reaches for.
Construction and principle
The standard gemmological refractometer consists of a high-RI glass hemicylinder, a calibrated reading scale, an internal optical system with eyepiece, and an integrated light source. The stone is placed table-down on the hemicylinder with a thin film of high-RI contact liquid (typically a 1.79 oil based on saturated methylene iodide) to optically couple the stone and the glass. Light from the internal source enters the stone at grazing incidence; rays meeting the stone-glass boundary above the critical angle for total internal reflection are returned, while rays at smaller angles refract into the stone and are lost. The boundary between returned and transmitted light appears as a sharp shadow-edge against the calibrated scale, readable through the eyepiece.
Range and limitations
The refractometer reads from approximately 1.30 to 1.81. The lower bound is set by the refractive index of the glass hemicylinder; the upper bound is set by the refractive index of the contact liquid, beyond which the optical coupling fails and the instrument cannot give a meaningful reading. Diamond, demantoid garnet, zircon, sphene, and synthetic moissanite all sit above the refractometer scale and require alternative methods. Within the working range, the refractometer measures RI to about 0.002 with practice, which is sufficient to distinguish most gem species and varieties.
The instrument requires a polished flat surface for accurate reading. Cabochons, rough, and stones with damaged or curved tables do not give clean shadow-edges; for these, the immersion method using calibrated RI liquids is the standard alternative. The contact liquid leaves a residue that must be cleaned from both stone and hemicylinder after each measurement, and the hemicylinder is fragile and softer than many of the stones placed on it — careless handling produces scratches that ruin readings.
Birefringence and optic character
For doubly refractive stones, the refractometer shows two shadow-edges. Rotating the stone through 360 degrees on the hemicylinder causes the edges to oscillate between maximum and minimum readings. The maximum and minimum define the two principal refractive indices, and the difference is the birefringence. By examining the behaviour of the two shadow-edges with a polarising filter (the polariser plate that fits over the eyepiece), the gemmologist determines optic character — uniaxial positive, uniaxial negative, biaxial positive, biaxial negative — which adds further diagnostic power.
Light source
Standard refractometers use sodium-vapour or sodium-equivalent LED illumination at 589 nm, the sodium D-line. Monochromatic illumination eliminates the chromatic dispersion that white light would produce at the shadow-edge and gives a sharp, repeatable reading. Older instruments used filament lamps with sodium filters; modern instruments increasingly use LED equivalents that approximate the wavelength without the heat or warm-up time of a vapour lamp.
In the trade
Refractometer readings are the foundation of every coloured-stone identification document. Buyers and dealers do not normally see RI values quoted on certificates, but every laboratory report rests on the measurement. For appraisers and trade gemmologists, the refractometer is the first instrument used after visual examination, and the readings dictate which subsequent tests — spectroscopy, fluorescence, microscopy — are warranted.