GIA Duplex I Refractometer (c. 1967)

The Duplex refractometer was developed by the Gemological Institute of America (GIA) under the guidance of Lester B. Benson in 1967. Benson’s innovation of the “spot” method for curved surfaces and the design of the Duplex system established a new standard for gemological measurement. 

The GIA “Duplex” Refractometer represents a major advancement in gemological instrumentation. Introduced as the first device capable of accurately measuring refractive indices on both flat facets and curved, polished surfaces, it provided gemologists with a versatile tool for identifying a wide range of gemstones. Its broad, stable base and elevated viewing height were designed to support precise measurement techniques in both laboratory and educational settings.

At the core of the instrument is a high-index lead-glass hemicylinder whose flat surface projects slightly above the instrument table. A refractive-index scale engraved on the side allows for direct readings, while an adjustable mirror—controlled by a knurled knob—provides consistent illumination through a small window in the housing. The Duplex features two eyepieces: one fixed and one sliding, enabling both distant-vision “spot” readings for cabochons and standard shadow-edge readings for flat surfaces. This dual-method approach made the instrument uniquely adaptable for different gem cuts and optical properties.

Over time, the Duplex became an essential instrument in gemology laboratories and classrooms, influencing subsequent generations of refractometers and solidifying GIA’s reputation as a leader in gemological research and instrument design.