An apparatus for applying a mark to a substrate, the apparatus comprising a housing (10) for containing a reservoir of marker fluid, a marker fluid retaining nib (12) protruding from one end of the housing and drawing marker fluid from the reservoir, the nib having a marking surface (18,20) contoured by one or more recesses (14) or protrusions, whereby, when the marking surface (18,20) is applied to a substrate an uneven distribution of marker fluid is deposited from the nib (12) onto the substrate resulting in a non uniform mark.