Being a high-Z material, mercuric iodide (HgI2) has a relatively high gamma-ray absorption coefficient. Its low charge carrier mobilities, however, have somewhat hampered the interest in using this material as a room-temperature gamma-ray spectrometer. By using the Frisch collar technology, the influence of the low charge carrier can be significantly reduced. The growth of HgI2 by the Faile method in a horizontal furnace fortuitously produces tetragonal prismatic crystals. These crystals with appropriate dimensions can be fabricated into Frisch collar spectrometers. With the Frisch collar technology, 1.8% energy resolution for 662-keV gamma rays has been achieved.