Electrum

Electrum was a natural alloy of gold and silver and to a minor degree of copper. The color depended on the mixing proportion of silver or gold.

Worth
Most coins in Faerûn worked 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 were worth 5gp, and platinum coins were worth 10gp.

Conversion
Simplified: 1000cp = 100sp = 10gp = 2ep = 1pp