# Some questions of precious metal Futures

We always regard precious metal as a FX rate e.g XAU/USD.

Is there any concept of spot rate for precious metal? If yes, then we know the dynamic of FX spot rate under risk neutral measure can be written as: $$\dfrac{dS}{S} = (r^d-r^f)dt + \sigma dW.$$ Here $$r^d/r^f$$ is domestic/foreign risk free interest rate. But for the precious metal e.g. XAU/USD, $$r^d$$ can be risk free interest rate of USD, what's $$r^f$$ here? Can anyone tell me the meaning in real market?