It looks like Constantine I from Constantinople, the mintmark is hard to make out but seems to be CONS[A]...I am not sure of the workshop. The bust is indeed rosette diademed.
Constantine I
A.D. 330-3
CONSTANTINVS MAX AVG rosette-diadem, draped and cuirassed bust right.
GLORIA EXERCITVS Two soldiers helmeted, stg. facing one another, reversed spear in outer hands, inner hands on shields resting on the ground; between them two standards.
in ex. CONS[A]
RIC VII Constantinople 59