Writing implement, in particular a pen

A writing instrument, comprising a body (1a, 1b) having a longitudinal axis and a housing (6) at least at one of its ends, the housing having a transparent outer wall (7), the height of this housing (6) between its bottom and said transparent outer wall (7) being substantially constant, at least one movable decorative element (9, 10) being movably mounted inside said housing (6), at least one of the dimensions of the movable element perpendicular to said height being substantially less than a corresponding dimension of the housing, characterized in that said decorative element (9, 10) comprises a lateral surface extending in the direction of the height of said housing (6), one end (10a) of this decorative element being shaped so that it contacts an adjacent wall of the housing (6) only in a zone situated in the center thereof to form a first guide means, whereas the other end is shaped to come into contact with an adjacent wall of said housing (6) by an edge (10b) of its lateral surface to form a second guide means, a small play being left between said housing (6) and the guide means, such that movement of this decorative element (9, 10) inside said housing is limited to a random movement in translation and in rotation about said central contact zone and means for removably joining the housing to the body.