How to account for precious metals (gold, silver, platinum, etc.)?

Which is exactly the same as share market companies listed on Stock Exchanges - without a fixed price, which changes from time to time (in fact daily).

Then treat their purchases as “investment” Inventory

Not exchange rates, you do market valuation adjustments as @danielgalindezm described with “you’ll have to calculate outside of Manager and record a journal entry to update the value for the whole ‘Inventory’ hold” - read post #4 in this topic for an example