The Diamond Clan (Japanese: コンゴウ団Kongō-dan, literally Diamond Gang), is an ancient tribe of people based in the Hisuiregion. They worship a deity they call "almighty Sinnoh", who created time. This has led to fighting with the Pearl Clan, who claim almighty Sinnoh created space. They are located at the Diamond Settlement.
The Diamond Clan first appeared in Pokémon Legends: Arceus. They have a history with the Pearl Clan, leading to several arguments and fights with the other clan, thinking they worship a false god. When the player first meets these clans' leaders, Adaman and Irida, they appear not to get along very well, relying on the Galaxy Team to keep the peace between the two clans. As a result of the space-time rift, the Diamond Clan's Noble Pokémon have become frenzied, causing destruction and harm with heightened power, rather than protecting the land they are to be guarding. The Diamond Clan also relies on the help of the Galaxy Team's Survey Corps, primarily the player character, to help quell the Noble Pokémon that the Diamond Clan is tasked to care for. Throughout the story, the Diamond Clan also provides the player with Ride Pokémon to help traverse the land of Hisui and quell the Nobles belonging to the Pearl Clan. The Diamond Clan becomes closer with the player character, trusting them with how they handle Pokémon.
After the player has been exiled from Jubilife Village, they ask Mai, warden of a special Wyrdeer in the Obsidian Fieldlands if the Diamond Clan is willing to take them in for shelter, declining their request at assumption it may cause conflict with the Pearl Clan or Galaxy Team, a situation they would all want to avoid. Both Adaman and Irida help the player in secrecy, along side Volo of the Ginkgo Guild. By the end of the game, the Diamond Clan learns to be in peace with the Pearl Clan and the Galaxy Team, with Adaman and Irida realizing the conflict between the two was really between themselves and not their clans.
Portraits of men who resemble Alder and Maxie are found within houses at the Diamond Settlement. The relationship between the two, if any, is unknown.