This report describes the occurrence of the Spruce Mine mica and its physical properties among which is an unusually wide range of colors. A table is presented showing the results of power-factor measurements made by the National Bureau of Standards on 196 samples of sheet mica from 109 mica mines and 15 feldspar mines. Samples from 71 of the mines have power factors within the limits allowed for use in radio-transmitter condensers. Other tables show the results of fabrication and electrical tests of samples from 10 mica mines in North Carolina, 6 of them in the Spruce Pine district, as determined by two commercial users of mica, who concluded that the mica in 8 of the 10 samples is suitable for use in condensers. By T.L. Kesler and J.C. Olson, 1942. 38 pages. Includes a map and a table.