AGS Shipley Diamond Colorimeter (c. 1960)

The 1960 Shipley Diamond Colorimeter represents a pivotal era in the transition from subjective "master eye" grading to objective, instrument-based diamond analysis. Developed for the American Gem Society (AGS) by Robert M. Shipley Jr., this unit holds immense historical significance as the first non-visual electronic colorimeter ever manufactured for diamond grading. Because production was limited and the technology has its weaknesses, very few units exist today, making this a remarkably rare artifact of gemological history that embodies the industry's early commitment to "Science over Sentiment."

The device operates on the principle of differential light transmission, measuring how much light a diamond absorbs across specific wavelengths—primarily the "yellow" end of the spectrum where nitrogen impurities are most active. To operate the unit, a technician first calibrates the device using a "blank" standard to establish a zero-point for 100% transmission. The diamond is then placed table-down in a light-shielded well, and a stabilized light source is passed through the stone. A photoelectric cell captures the exiting light, converting it into an electronic signal that is displayed on a galvanometer needle-gauge, which corresponds to the standardized AGS or GIA color scales.

Despite its revolutionary status, the 1960 Colorimeter possessed technical weaknesses. A primary limitation was its sensitivity to fluorescence; the blue emission from fluorescent diamonds could "trick" the photoelectric cell, leading to inaccurate readings of the stone's true body color. Additionally, the device was susceptible to geometric interference, where the specific cut or proportions of a diamond could cause light leakage or internal reflection. This colorimeter could not rotate and average the color of diamond over 360° as well.  Modern patents have since solved these early hurdles by utilizing integrating spheres and multiple light sources to ensure a level of accuracy that this analog pioneer first made possible.