This is a Nursing Officers breast badge of the St John Ambulance Brigade, used sometime between 1887 - 1974. It is the No. 3655 Breast Badge, and was worn by the nursing division officers on the left side of the chest, usually positioned above a breast pocket.
Black circular badge featuring a white embroidered 8-pointed cross symbol. Around the edge of the badge in a circle, the text "THE ST-JOHN-AMBULANCE-BRIGADE" is embroidered, and around that, again in a circle to enclose the text and the St John Ambulance logo, are two laurel leaves, meeting at the bottom.