Its in pretty bad shape (looks like its been through a rock tumbler!) but it is 100% authentic and identifiable.
It took me a while but I can just about make out the letters in the left and right fields, '
S' and '
F'.

This means the coin had to have been made in Trier (Germany), its mintmark is '
PTR', the '
PT' is pretty clear when you know to look for it but the '
R' has been squashed beyond all recognition.
Your coin is
RIC VI Trier 602a, it would have been minted between 303 and 305AD, here is a better quality example:
http://stampcircuit.com/Auction_Ite...1967?page=14
Obverse CONSTANTIVS NOB C - Llaureate, cuirassed.
Reverse GENIO POPV-LI ROMANI - Genius, naked.
S - F in fields, PTR mintmark.At the time this coin was made Constantius was a Caesar and ruled over Britain, France and parts of current day Germany.