Measurements of the magnetic properties of four gem-quality synthetic diamonds indicate that higher levels of permanent magnetism can be induced in the synthetics than could be induced in a number of gem-quality natural stones. As received, the colored and included synthetics showed much higher levels of magnetism than the natural stones, but one near-colorless synthetic had less magnetism than many of the natural stones. Most of the magnetism of the natural stones is due to surface contamination, which can only be removed by rigorous acid cleaning. After cleaning, the natural diamonds were not only less magnetic than all of the synthetic stones, but they were also more resistant to demagnetization. Significant levels of metallic impurities were detected in most of the synthetics. These results on a limited number of samples suggest that natural diamonds can be distinguished from synthetics on the basis of their magnetic properties, but that in the case of near-colorless, inclusion-free stones, such tests may require sophisticated instrumentation.