The Diamond Medal is awarded by completing any track on Impoppable Difficulty. You can also get Bronze Medals, Silver Medals, and Gold Medals. The Diamond Medal is the highest possible medal in BTD5 Mobile and Bloons Adventure Time TD, excluding Mastery Mode medals. Diamond Medals look like a round black medal with a monkey head on center with diamonds around the edge.
In Bloons TD 5 Mobile, every Diamond Medal achieved allows 1/4 of the way of completing all medals of each track. In addition, getting a Diamond Medal contributes part of the progress for unlocking Mastery Mode.