Abstract: A method and apparatus for determining the inert content of a coal dust and rock dust mixture uses a transparent window pressed against the mixture. An infrared light beam is directed through the window such that a portion of the infrared light beam is reflected from the mixture. The concentration of the reflected light is detected and a signal indicative of the reflected light is generated. A normalized value for the generated signal is determined according to the relationship.phi.=(log i.sub.c `log i.sub.co) / (log i.sub.c100 -log i.sub.co)wherei.sub.co =measured signal at 0% rock dusti.sub.c100 =measured signal at 100% rock dusti.sub.c =measured signal of the mixture.This normalized value is then correlated to a predetermined relationship of .phi. to rock dust percentage to determine the rock dust content of the mixture. The rock dust content is displayed where the percentage is between 30 and 100%, and an indication of out-of-range is displayed where the rock dust percent is less than 30%.