I haven't found a really satisfactory solution to this problem, but I can get close. With my current setup, I can alter the camera setting from my computer and take two photos with different exposures without moving the coin or the camera even a little. You can also do this with a
good tripod or camera stand and a remote trigger, cable or wireless, if you are careful when changing the camera setting.
One shot will be exposed to capture some detail from the brightest areas, and the other to make the rest of the coin look silvery.
Then I use Photoshop to
overlay just the detail from the bright areas, from the first shot, onto the second one. This is why you need absolutely no movement between the shots.
Not all silver coins need this. With most of them, I can find detail in a single shot and make sure it is
still there after processing the photo to bring out the coin's silveriness. The coin does not need to look bright to look like silver. I think this example does quite well, and actually a lot of it is quite dark.
Bill