We only have estimates for unknown discoveries, and the margin of error is huge for those. Not well known at all. There is definitely a finite amount of gold in the crust, but no one knows how much, and even "knowing" how much China has locked up in their vaults is difficult (they keep it secret for obvious market manipulation reasons). Of course, we are less than a hundred years away from being able to mine asteroids, but I'll doubt we'll bother mining for something as intrinsically worthless as gold.

The US dollar is still a very stable currency, whatever the libertarian alarmists claim.