Melee-size yellow synthetic diamonds have recently been seen in the gem market, mostly as loose stones but sometimes set in jewelry. The material identified to date is synthesized by the high-pressure, high-temperature (HPHT) process. Randomly occurring pinpoint or flux-metal inclusions are diagnostic identification clues. However, some synthetic diamonds require advanced laboratory methods. This report describes the effectiveness of a technique new to gemology, infrared microspectroscopy, in combination with cathodoluminescence imaging for the rapid and accurate separation of melee-size natural and synthetic diamonds based on their characteristic mid-infrared absorption spectra and growth structures, respectively. With these techniques, the GAAJ-Zenhokyo Laboratory in Tokyo determined that approximately 10% of loose—and dozens of mounted—yellow melee diamonds submitted over a four-month period were synthetic.