You go to Canalave City which is west of Jubilife City. You will need surf to get there.
The 8th Badge in Pokemon Diamond is called The Beacon Badge.
you get it l8ter in the game after u beat the water badge i think its the 6th badge
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You can get the 6th badge by defeating your father in petalburg.
In Diamond, Badge 5 can be gotten by defeating Fantina in her Gym in Hearthome City.
Hearthome
hearthome city
The lose of dignaty you had
Hearthome City.
the seventh gym badge snowpoint gym
go to eterna city
you beat everyone in the gym