Platinum

Platinum is a rare silvery-white metal sometimes used as a currency in the Realms. Its value is generally greater than gold and, like gold, it's resistant to oxidation.

Worth
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 sp, 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).

Conversion
Simplified: 1000cp = 100sp = 20ep = 10gp = 1pp

In the 1st edition of the Forgotten Realms Campaign Set the currency is based on a gold standard but the conversion to other coinage is different:

200cp = 20sp = 2ep = 1gp = 1/5pp

or with fractions removed:

1000cp = 100sp = 10ep = 5gp = 1pp

Note that electrum was valued less than gold. This system of currency is/was used in the nation of Cormyr and many other organized nations.

Uses
Elves of Myth Drannor used platinum to craft enchanted +5 arrowheads.