Dumbledore's Army coin

"You see the numerals around the edge of the coins? On real Galleons that's just a serial number referring to the goblin who cast the coin. On these fake coins, though, the numbers will change to reflect the time and date of the next meeting."

- Hermione Granger

The Dumbledore's Army coin was a fake Galleon created by Hermione Granger in 1995 to inform members of Dumbledore's Army when the next meeting would take place.

Creation and usage
As it would become more suspicious for members to keep meeting each other in groups in the halls to set up the time, Hermione fabricated fake Galleons, on which she then placed a Protean Charm. Around the edge of each coin was a series of numerals which, on genuine galleons, represented a serial number belonging to the Goblin that cast the coin. The Protean Charm allowed these numerals to change into the time and date of the next meeting of the D.A. whenever the master coin, owned by Harry Potter, was changed. The coin would also warm up to alert the holder to the change.

Hermione stated that she got the idea from Voldemort pressing the Dark Mark on the arm of his Death Eaters, summoning them. However, Hermione chose to engrave the date on the coins, rather than on the members' forearms. Harry agreed that this way was preferable.

During Draco Malfoy's mission to assassinate Albus Dumbledore, he was inspired by the D.A.'s coin idea, and created his own coins to communicate with Madam Rosmerta, on whom he used the Imperius Curse, to help with his mission. Around that time, Harry instructed Hermione to use their version of coins to recruit any Dumbledore's Army members to prepare themselves against Malfoy; only Neville Longbottom and Luna Lovegood responded, as they were the most vigilant members.

In 1998, Neville Longbottom used the coins to summon former members of Dumbledore's Army to Hogwarts to fight the Death Eaters and reclaim the school. The former members alerted many others, including the Order of the Phoenix, and set the scene for the Battle of Hogwarts.

In later years, the surviving members of Dumbledore's Army would keep their coins, regarding them as badges of honour. It is unknown if the two defectors, Marietta Edgecombe and Zacharias Smith, kept their coins or not.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows