This study investigated the effectiveness of a handheld magnet in detecting synthetic diamonds. A total of 104 synthetic diamonds from nine different manufacturing sources were tested. Of the HPHT-grown synthetic samples tested, 58% registered a detectable magnetic response. Strong N52-grade neodymium magnets were able to detect 35% more HPHT synthetics than traditional ferrite and alnico magnets. CVD-grown synthetic diamonds showed no magnetic attraction. A set of 168 natural diamonds, including HPHT-treated natural diamonds, was tested as a control group. None of the natural transparent diamond samples showed attraction to a neodymium magnet, but two heavily included translucent specimens with unusually large opaque inclusions were strongly magnetic. A new magnetic testing method is presented for detecting small magnetic inclusions in loose HPHT-grown products, and for identifying small synthetic diamonds mounted in jewelry.