the ground gym
Beat the gym leader
stoopolis city
Beat the 8th Gym leader.
The 8th Badge in Pokemon Diamond is called The Beacon Badge.
earn it.
at the gym
Sootopolis City. It is accessible only by using dive. The eighth badge can be obtained after the scenes with Rayquaza. The ice puzzle is fun, by the way.
Go to the Pokemon league.
beat the #?*@! gym leader
beat the league
== == You get the 8th gym badge in Sunyshore City. Hope this helps
Dewford Town.