Troy weight

[shikamaru]

So a one-ounce coin is a Troy Ounce? - Always?

A coin will be measured in troy weight, yes.
Will it always be one troy ounce exactly? No.

Example
An American Silver Eagle is exactly 1 troy ounce.
An America Gold Eagle is a little bit more than 1 troy ounce, but will be said to be a troy ounce.

The weight, purity, and content of the coin depends on the issuer of the coin.