Make gold coins out of 50% gold and 50% (pick your mettle) and put a fancy stamp on it and there you go. Counterfeit.
I still don't see the big profit in making counterfeit septims when I can smelt the gold ore into ingots, make jewelry, enchant it out the wazoo and then sell it for a great big pile of shiny shiny real ones. I guess if you're not doing any enchanting it makes a difference but even unenchanted gold jewelry goes for a pretty decent price if you find the right gemstones while you are mining all that iron ore (or off looting various dungeons).

