Say I were to loan an ounce of gold to someone. What incentive would I have to do so except for the anticipation of the return of a greater quantity of gold? Likewise for silver or fiat currency.
Interest is necessitated on account that debtors are humans, i.e. they have the uncanny tendency to do stupid things with money or act with malfeasance and defraud, both human attributes. Thus, all other things being equal, if 3% of my gold is defaulted upon, I would have to charge approximately 3% interest just to break even.
That said, if interest is prohibited, there will be no loans, and one of two things will happen. Either those who require credit will do without, letting the economy suffer, or government will simply print money on its own, which will cause rampant inflation as government throughout history has been careless with its currency. From the Ming Dynasty to the more recent example of Zimbabwe, any time (un)elected officials have access to printing presses, they act irresponsibly. That said, 'everything is worth what the buyer will pay for it'; if they wish to use gold, let them use gold; if they wish to use silver, let them use silver, if they wish to use warehouse certificates for wheat even, let the marketplace decide the yardstick with which it arbitrates.
That said, in lieu of the 'goldsmith' model, one would simply have to demand a certificate certifying that in his safe rests a quantity of gold with one's name on it. If there is not, that is called 'fraud'. If one wishes to have one's gold lent out, one is certainly entitled to a cut of the interest, otherwise what is preventing one from keeping it safe for one's own use, as surely that is safer than if it is lent out?
At any given time, I keep no less than 10 oz. gold and 500 oz. silver stored in my residence, automobile, or both, since I trust in 5'000 years of precious metals as a store of value much more than in any country's fiscal restraint. Hitler would have not liked this, as he prohibited ownership of more than 5gr gold, for use only on rings...tyrants dislike precious metals as their control over the economy ensures control over the people.
Finally, I do not loan for less than 20% compound interest, what with the credit crunch and all. It used to be 15%