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Platinum is a rare silvery-white metal sometimes used as a currency in the Realms. It's value is generally greater than gold and, like gold, it's resistant to oxidation.
Most coins in Faerûn work off of the silver and gold standard, with copper pieces (cp) being worth 1/100th of a gold piece (gp) and Silver pieces (sp) being worth 1/10 of a gp. electrum coins are worth 5 gp, and platinum coins are worth 10 gp.
Each coin is approximately 30.6 mm in diameter and weighs approximately 9 grams each (or 50 to one pound).
Simplified: 1000cp = 100sp = 10gp = 2ep = 1pp
200cp = 20sp = 2ep = 1gp = 1/5pp
or with fractions removed:
1000cp = 100sp = 10ep = 5gp = 1pp