C-Mart, you are very correct that many Roman issues from around this period bore the letters SM for Sacra Moneta in the exergue. Your reading of the exergue still leaves the mint mark ambiguous, however. It could well be 'SARL' for the second workshop at Arles, or 'SMK Gamma' for the third workshop at Cyzicus. If the obverse figure is indeed Constantius II, then the 'SMK Gamma' reading is correct, since the issue from Arles only exists for Constans (The Constantius II issue has a star or a palm branch after 'SARL').
But that's all some very detailed technical talk! These cool old coins were struck by the ancient Roman Empire in the names of Constans, Constantius II, Constantius Gallus and Magnentius. The front ('obverse') has the bust of the Emperor, with his name and titles around. The reverse shows the emperor standing left on a galley, holding either a small Phoenix or a Victory on a globe, and a labarum (military standard) in the other hand, with Victory sitting at the back, steering the galley. The scene is thought to depict Constans crossing the sea to Britain in 342 AD. This may explain why Constans himself favored these coins over the 'fallen horseman' type which bore the same legend. FEL TEMP REPERATIO translates roughly into 'restoration of happy times'.
As with many Roman coins, there are hundreds of different types. On the obverse, you have different emperors with different titles and headgear. On the reverse, you have different mint marks (found in the 'exergue', the few letters below the imagery on the reverse), along with different letters found in the field. It's outside the scope of CoinQuest to list every single type with value. Instead, we will call out general values for common coins, and list some interesting rare and valuable issues.
CONSTANS AND CONSTANTIUS II (COMMON COINS):
worn: $6 US dollars approximate catalog value
average circulated: $30
well preserved: $75
CONSTANTIUS GALLUS (COMMON COINS):
average circulated: $60
well preserved: $120
average circulated: $180
well preserved: $450
There are different size categories for these coins. Some are 18-19 millimeters in diameter, some are around 21-22 mm, while others are 24 mm or even larger, with some coins being up to 26 mm in diameter. Generally, the bigger the coin is, the fuller the inscriptions are. Larger coins often show off the design better, and even have a circle of beads around the entire design on both sides. Collectors prefer the bigger coins!
'Larger coins' below refers to coins larger than 22 mm in diameter.
CONSTANS AND CONSTANTIUS II (LARGER COINS):
average circulated: $50
well preserved: $150
Coins in the name of Constans or Constantius II where the emperor is portrayed on the obverse *holding a globe in his hand* are scarce. They come with 'TS epsilon star' or 'TSA star' in the exergue. For one of these, multiply above values by 3.
CONSTANTIUS GALLUS (LARGER COINS):
average circulated: $100
well preserved: $200
Coins in the name of Constantius Gallus with a *star* in the field on the right side of the reverse, and 'R epsilon', 'RS', or 'RZ' in the exergue are big, bold, and scarce! If you can find one of these, multiply the above values by 5.
Some coins, limited to well preserved examples, have survived with their original thin layer of silvering on the surfaces still intact. Such examples are the most sought-after by collectors, and command premiums. For any coin with a nice layer of silvering still visible, multiply the appropriate catalog by 1.5.
As with all ancient coins, value is hugely dependent upon a number of factors:
Better coins have intact edges and rims, are sharply struck, well centered, with legible inscriptions and a clear exergue or other mint marks.
Worse coins have corrosion damage, are poorly cleaned, are chipped or broken, have unclear inscriptions and ambiguous lettering in the exergue.
Please read our Important Terminology page for interpreting catalog values.