Re: Gold crashing (a little anyway)
I think it's mostly density of value. A small amount of gold has a lot of value. The value tends to be relatively stable over time. And unlike things like gems, it's easily workable, reworkable, and divisible. Paper money is more portable still, but opens itself up (as we have seen) to abuse.
If you use paper money backed by a specific commodity, anything storable and valuable would probably work, even if it was bulky.