The Treasure Mark is a glass bangle that belonged the infamous Captain John. It's been the desire of Captain Buggy for years, and it leads to an amazing treasure.
The Treasure mark of Captain John
The Treasure Mark originally belonged the pirate Captain John. At some point his body was claimed by Gecko Moria and gave it to Doctor Hogback to change into one of his General Zombies. It's unknown if anyone realized the significance of the treasure mark. Though, it was put in the treasury to Thriller Bark.
The Treasure Mark was eventually found among the treasure stolen on Thriller Bark. As Luffy was going through the pile of treasure Nami was enjoying. This glass bangle caught his attention. Usopp tried to warn Luffy about taking treasure from Nami, but she didn't care since it was glass. She had no idea, and neither did Luffy, that this would mark the location of a treasure.
In Impel Down
Buggy barters for the Treasure Mark
When Luffy ran into Buggy in great Marine prison Impel Down. Buggy recognized the item as the one he has desired for years and realized that Luffy had no idea what it was he was wearing. Luffy was looking for Ace deep within the prison. Buggy was trying to get out. Buggy tried to barter his assistance to Luffy in exchange for the bangle, but he didn't want to give it up so easily. Calling it a gift from Nami. Eventually, Luffy agreed to handed it over to Buggy when he agreed to lead him to the deeper levels to find Ace. Even in spite of Buggy accidentally slipping that this glass bangle was a map to a great treasure. Luffy happily handed it over. Buggy was overcome by Luffy's kindness and honesty. Buggy had intended to abandon Luffy as soon as he got the Treasure Mark, but due to events. Buggy had no choice but to join Luffy deeper still into the jail.
The treasure mark is not seen until three weeks after the war with the Whitebeard Pirates and Marines. Once Buggy is free to leave. He and Mr. 3, who formed a partnership during the escape from Impel Down, are able to find Buggy's original crew. On board the Big Top, Buggy's pirate ship, he showed Alvida the key to the treasure of Captain John.