This mineral is found embedded in small crystals in prehnite, at Bishop town, in Renfrewshire. It had been long supposed by mineralogists to be a variety of zinc-blende; but it was first distinguished from that mineral by Lord Greenock, who communicated his opinion to Professor Jameson; and two small crystals, together less than a grain in weight, were sent to me for chemical examination by the latter, who concurred in the supposition that it was a new mineral.