This is a highly vitreous transparent reddish brown mineral, which occurs with quartz and rhodocrosite. Its H. is 3.5, and sp. gr. only 2.49.

When heated in a tube closed at one end it gives off water, but is otherwise unchanged. More strongly heated in platinum forceps it loses colour and very slightly fuses on the edges.

It is brittle, and has a highly conchoidal fracture. When digested with hydrochloric acid, oxide of manganese is dissolved out, and the silica which remains is perfectly colourless.