Island Medallion

The Island Medallion, or simply called Medallion, is a medal given to you as a reward if you complete an island/quest. The Medallion comes with 50 free credits (it was originally 100), and can be worn by the Poptropican.

Islands
The only way to earn the Medallion on each Island is to complete the quest. Each island has different ways to complete quest. Here's a description on each island's main quest:

Previous Owners
All the medallions belong to someone else before you got them.

Early Poptropica Island: The Chief of Early Poptropica

Shark Tooth Island: Professor Hammerhead

Time Tangled Island: Your future self

24 Carrot Island: The Mayor

Super Power Island: Ned Noodlehead/Chief of Police

Spy Island: Top Secret Secretary

Nabooti Island: Woman at museum

Big Nate Island: Mrs. Godfrey

Astro Knights Island: The King of Arturus

Counterfeit Island: Policeman

Reality TV Island: The Helicopter Guy

Mythology Island: Athena

Skullduggery Island: Governor Roland

Steamworks Island: Mayor Crumb

Great Pumpkin Island: Lucy

Cryptids Island: Harold Mews

Wild West Island: Marshal Taylor

Wimpy Wonderland Island: Greg Heffley

Red Dragon Island: Jack and Annie

Shrink Ray Island: C.J.

Mystery Train Island: Grover Cleveland

Game Show Island: Dr. Harold Langley

Ghost Story Island: Magistrate

S.O.S. Island: Captain Boomer

Vampire's Curse Island: Count Bram

Twisted Thicket Island: Elf Queen

Poptropolis Games Island: MC

Wimpy Boardwalk: Greg Heffley

Lunar Colony Island: Director McNabb

Super Villain Island: Security Guards

Charlie And The Chocolate Factory Island: Mr Willy Wonka

Zomberry Island: Samuel Brains

Night Watch Island: Slugger's Owner

Back Lot Island: The Movie Manager

Virus Hunter Island: Dr. Lange

Back Lot Island:  Carson Willis

Mocktropica Island:  Woman working at Poptropica headquarters

Monster Carnival Island: Edgar

Credits
Completing Islands in Poptropica are the easiest and simplest way to receive credits. When the player completes an Island, they will receive 100 credits to spend in the Poptropica Store. However, since late 2011, completing an Island will only award you 50 credits.

Restart
Starting July 6, 2011, Poptropica started allowing players to replay islands, while keeping all the Island Medallions they had already earned. The medallion they received from completing once will disappear from their inventory, but not their friend's page. However, they will not receive credits.