Copper-based alloys were the primary metals for making small denomination coins in the Roman era. In terms of uncleaned coins it is rather out of the ordinary to come across one that isn't. Surprisingly, pure copper coins are rare. The Roman mints freely mixed in other metals to make the blanks that would become coins once die-struck. When copper is alloyed with tin bronze is created; the favored alloy for late Roman coins. Although the ratio of tin and other trace metals varied, bronze is still predominantly a copper mixture.Bronze coins sometimes look coppery, especially when the coin's metallic surface is revealed. Most often however bronze will form a thin layer of oxides that take on various colors ranging from greens to browns to pitch black.
Early in the empire a curious ore was found that when mixed with copper turned the metal golden. Although the ancients didn't understand that this was due to the element zinc they did note the desirable effect and used it extensively. They referred to this alloy as orichalcum. Brassy coins may initially be confused with gold before a closer look reveals their humbler nature. One may come across these orichalcum coins fairly frequently but few will be very detailed or have survived in collectible shape. Sadly, there are a number of disreputable dealers who attempt to pass off these ordinary slugs as real gold coins. Caveat emptor!
Also to be considered copper are the coins featuring emperors with radiate crowns popular from the mid-250's through the early 300's. These copper-based coins, also referred to as Billon, a non-precious metal alloy with trace amounts of silver, received a thin silver plating at the mint and were officially regarded as real silver coins. Nobody was fooled and inflation ensued having a disastrous effect on the Roman economy. Silvered coins rarely retain their silvering intact unless they were buried in pots while still being relatively new. Even then it only took minor circulation for the silver to wear through on the high points of the coin such as in the example on this page.
True silver coins will look silvery even when very worn. Sometimes they are recovered still looking "new" even after thousands of years of being buried. Usually, however, they have tarnished and some have even become as encrusted as bronzes can. For this reason you have at least a slight chance of finding a real Denarius underneath a lot of dirt that is hiding that silvery glint. Statistically, this happens in about 1 in 100 to 1000 coins depending on where regionally that particular batch of uncleaned coins came from.
Gold coins remain the holy grail for any coin collector. You can forget about finding one mixed in with other uncleaned coins. It won't happen for the simple reason that gold does not rust and is to a certain degree "dirt repellent". Quite unlike a nearly worthless bronze coin, the few that Romans came across they guarded with their lives. No one casually drops a gold coin on the ground and so you can expect that their loss was always a rare event. Fast forward a thousand and a half years later and take into consideration that even a beat up 1-gram Tremissis is worth well over $100 for the most common of types and you have really stacked the odds against coming across one. The best hope of finding a silver coin is when they're completely covered in tough soil. This not being an option with gold the chances hover near zero. Sorry, we'd all like to hit the jackpot and it's worth dreaming about perhaps but, realistically, don't hold your breath!