GIA Erb Gray Refractometer (c. Early 1960s)

This early-model GIA Erb Gray refractometer from the Gemological Institute of America features a turnable hemispherical prism, mounted on a rotating optical turret. By rotating the hemisphere, the user can perform a 360° refractive Index examination without turning the stone itself - and potentially scratching the soft hemicylinder/ hemisphere glass. The marking on wheel gives the exact angle the stone turned without guess work.

The instrument’s heavy-duty cast housing, textured enamel finish, and flip-up prism cover were designed for protection from damages. A precisely ground hemispherical prism, paired with GIA’s early illumination channel, allowed accurate measurement of refractive indices from common gemstones. Later GIA models has removed this turnable hemisphere and made a cost-effective, more compact designs, this early hemispherical version marks an important developmental step—illustrating how to adapt scientific principles into practical tools that shaped the foundations of modern gemstone testing.

This is a museum archives and no unit is available for sale at this moment.