echizento is correct.
The coin has a very nice green patina.
I found this site very useful when I was starting out hidden:
http://www.forumancientcoins.com/dane/It has downloadable Excel spreadsheets with lots of examples on them.
If you also use the filters in Excel you can usually 100% ID a coin quite quickly.
The sheet for this type of coin - a 'SECVRITAS REIPVBLICAE' can be found at the bottom of the page under the heading 'Victory advancing left'. Do not confuse this with another sheet near it called 'Victory standing left'.
Simply download the sheet and filter it for the emperor 'VALENS' and its mintmark which I think is 'ASISC'