Hi, I am New here fresh on today I have 30+ Years of Gaming behind me and hopefully many more ahead of me.
Nickinroch: It really depends on the tools, Traditionally it's a file or knife and they simply cut or grind off about 1/10th of the weight of the coin, The thief could with the correct skill checks do 1d10 per turn, 6d10 per hour or 48d10 per day, provided he has that many coins this will get him 1/10 that number in gold alloy (commonly gold & tin mix) because all gold coins are not pure gold, its too soft to hold the embossing. That gold would then need to be smelted and he would lose 1/2 in the process (no less than 1/4) and get gold nuggets.
Also, most good merchants have scales and will weigh coins! Especially if they have shiny edges or look off to them. If he gets caught they may charge him more (Make up the weight) or quietly send the bellhop for the local guards for the reward for turning in a counterfeiter/ Defacer of currency (Normally treated as HIGH Crimes in most coin based societies). Hope this helps.