This is a very crude way to discuss. If you'd read Rothbard you will see he never talks about some fiat-currency but gold ounces and that even those earn their interest. That it's not so with current gold is simply because gold is calculated in whatever fiat-money you can think of. And you do...
It's quite interesting to see, what place property rights play in any country. There are countries where this is values really high, but there also are countries in which grants on property rights look like an "oversight" or afterrthought. Let's take Germany, we are infamous for a few...