Golden drachma

A Golden Drachma is a large round coin that can be made up of pure gold or silver that was used as currency by the Ancient Greeks. However as, stated in The Lightning Thief, the gods never use anything less than pure gold. Mortals used to use the silver drachmas. They are described as big as Girl Scout cookies by Percy and are used by the gods and demigods. The drachmas have images of various gods stamped on one side and the Empire State Building on the other.

Uses
The ancient mortal drachma was made from silver, but in the Percy Jackson Universe the Gods now use nothing less than pure gold to forge them (the gold is mined from Olympus). Golden drachmas are used to send Iris messages.

Percy Jackson and the Olympians
A drachma can be used for the following: Currently, the pure gold drachma is the main currency of the gods, goddesses, and demigods.
 * Create an Iris Message
 * As payment to get into the Underworld
 * To pay the Gray Sisters to take you anywhere as long as its within their serving area
 * Payment for sending a letter or package along the Hermes Express

The most common use for the drachma is to send an Iris message through the rainbow goddess. It is used for contact when nothing else is available, and is quite useful if any extra drachmas are allowed for use.

The Lightning Thief
After beheading Medusa, Percy Jackson fills a pouch with drachmas and mails the head to Olympus. He later uses some drachmas to bribe Charon when he first enters the Underworld.

The Son of Neptune
Roman demigods don't use drachma's because it's Greek currency. Instead they use denarii. The Romans never used Iris Messages like the Greeks because they relied more on their giant eagles and road systems.

The Lightning Thief
Percy, Grover, and Annabeth find a fountain filled with drachmas at Aunty Em's Garden Gnome Emporium which they later use to pay Charon after their mortal money was burnt by him.

The Lightning Thief
Drachmas aren’t used as currency, instead they are used as equipment for characters. They increase certain attributes to whoever equips it. Each character can equip two. There are 20 drachmas that the player can collect in the game.

Trivia

 * Drachmas are Greek currency, while Aureus or denarii are Roman currency.
 * The word drachma came from the word drássomai, which means "to grasp."
 * The Drachma was the oldest currency in existence until Greece adopted the Euro.