Not rare, rated scarce by RIC but they seem to be relatively easy to get hold of. This one is not in good condition but was inexpensive and a Mars coin which is one of my colleting themes. It was a lone metal detector find in Yorkshire. Constantine the Great - Half Follis Obv:– CONSTANTINVS AVG, Laureate, cuirassed bust right Rev:– MARTI CONSERV, Mars helmeted, in military dress, spread cloak, standing, right, holding reversed spear, left hand leaning on shield Minted in Trier (// PTR). A.D. 310 - 311 Reference:– RIC VI Trier 897 (S)
I like Mars reverses too. Here's a Mars reverse I have a photo of: Maximinus II, Daia, AD 308-313 AE follis; 6.2g, 22mm, 6h; Cyzicus mint, AD 308-309. Obv.: GAL VAL MAXIMINVS NOB C; laureate head right. Rev.: VIRTVTI EXERCITVS; Mars advancing right, naked except for floating chlamys, holding transverse spear, trophy over shoulder. Delta in left field. // MKV Reference: RIC VI Cyzicus 49, p. 587.
Fractions are not really common. The OP one is probably around 2g, 3 years earlier, a 1/2 follis from Trier was 5g. This one is issued by Constantine as Caesar for a very brief period, as Constantine always hated his confinement as "just" Caesar: CONSTANTIN I AE22mm 5g Half-Follis/Nummus (aVF, some pitting) AV: FL VAL CONSTANTINVS NC; cuirassed laureate bust r. REV: PRINCIPI IVVENTVTIS; Emperor standing facing, head left, in military dress holding standard in each hand. S - A in fields l. and d. EXE: PTR Trier mint. REF: RIC VI Trier 743, rated R2, summer 307AD. Scarce denomination for Constantine as Caesar, minted only for a short period in the summer of 307AD.
Not a half follis, but from Trier with a Mars reverse. CONSTANTINE THE GREAT AE Follis (Reduced) 3.8g, 19.8mm Trier mint, AD 316 RIC VII Trier 112 (R5) O: CONSTANTINVS AVG, laureate, draped and cuirassed bust seen from behind, right. R: MARTI CON-SERVATORI, Mars helmeted, in military dress, cloak spread over chest, standing right, holding reversed spear, left hand on shield; T and F in field, ATR in exergue.
