The eighth gym is in Sootopolis city which is near route 126 and 127 (you have to dive to get there). First you have to find Wallace in the Cave of Origins in Sootopolis city and tell him that you think Rayquaza is in the Sky Pillar. Then you have to find the Sky Pillar (route 131 through a space in the rocks) and battle Rayquaza. It will then fly away. You then go back to Sootopolis city where Rayquaza will stop Kyogre and Groudon fighting. Then you can go to the gym there(:
stoopolis city
Try and get the 5th one lol.
It depends which ones are you looking for. The 8th Badge is in Sootopolis, The 7th badge is in Mossdeep, The 6th Badge is in Fortree, The 5th badge Is your dad in emerald it is found at Petalburg, the 4th Badge is in Fallabore, The 3rd badge is in Mauville, The 2nd badge is in Dewford, The 1st badge is in Rustboro.
at the gym
In the heat gym
the ground gym
stoopolis city
Beat the 8th Gym leader.
Beat the gym leader
Try and get the 5th one lol.
The eighth badge can be obtained by defeating the gym leader in viridian city.
Beacon Badge
After getting all the badges the 8th gym will open You beat givoanniORWalk Through Walls!!!!!!!!!!
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.
they will be on the montain, once you have obtained the magma badge
The 8th Badge in Pokemon Diamond is called The Beacon Badge.
I get my 8th badge in Sootopolis gym.