During the summer of 1913 the Royal Ontario Museum of Mineralogy secured a nmnber of samples of ruby-silver from the collection of a deceased mining engineer who had been employed in the Cobalt district. Judging from their association it is presumed that they were obtained from the O'Brien mine. The crystals for the most part are less than two millimeters in length and very few exceed a millimeter in diameter. They are light ruby-red in colour and exceedingly brilliant, and casual inspection suggested that they were proustite. As this mineral had not been described from the Cobalt region, it seemed desirable to confirm this supposition by chemical analysis and crystallographic measurement.