Pure gold is relatively "soft" as far as metals can go, and can be marked with the pressure of your fingernail. The "4-9" or 99.99% pure gold refered to in Umineko is rather close to this, so while no one could bend an ingot with their bare hands, one could assume that after working it awhile with raw strength, Rosa could, in fact, do this. After a week or so. 7*24 hours straight. Like a machine.
