When Isaac Newton first held a prism up to the sun, he scarcely could have imagined the influence of the spectroscope on modern gem testing. In the early 1950s, GIA’s Robert Crowningshield became expert at using this instrument to identify gems, including enhanced and imitation goods. This article shares Crowningshield’s methods, including several original drawings of spectral bands that reveal how certain gemstones selectively transmit or absorb light, yielding valuable clues to their identity.
