I am not sure if you used this, but central spot metering might improve the quality of the details.Usually on full auto the camera chooses a matrix metering which is influenced by the dark background and wich determines the blown highlights in the coin. Using center spot metering will usually expose better the coin itself, rather than the whole picture.
You can also try this trick: set AV mode instead of auto and select the smallest number available (2.8 or 3.2or whatever it is).