Enchanted coin

Enchanted Coins/Galleons are used for communication. Like real Galleons, the coins have numerals around the edge; on normal Galleons these serial numbers indicate which goblin cast the coin, but on the enchanted coins, the numbers represent the time and date of the next meeting, and change automatically to match whatever numbers the user sets on his coin. The coins become hot when the numbers change to alert the members to look at their coins.

Apperance
In Harry Potter and the Half-Blood Prince, Draco Malfoy uses a pair of enchanted coins to bypass the communication limits then placed around Hogwarts, thus managing to keep in contact with Madam Rosmerta, whom he had placed under the Imperius Curse. Draco reveals he got the idea from Hermione's DA coins, which were themselves inspired by Lord Voldemort's use of the Dark Mark to communicate with his Death Eaters. They are also used to alert Luna Lovegood and Neville Longbottom that the Death Eaters have entered the school. In Harry Potter and the Order of the Phoenix, Hermione Granger creates fake, enchanted coins that are used for communication between members of Dumbledore's Army.