What's heavier, a pound of adamantine or a pound of mithril ?

[jasper]

...If that is the case then a pound is not a pound. The unit of precious metals is the Troy Pound and 1 pound = 1.25 Troy pounds, so a pound of Anamantine would weigh more than a pound of Mithril in lands where Mithril is a precious metal and not an ordinary metal.

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Um where in the 3E does it mention Troy oz? So a pound is pound is pound.
Also can't a dm just make up the price to reflect how rare admantine is?