He is the first Gym Leader you meet and he is in Oreburg City, through the passage east of Jubilife.
At the very bottom.
I think you get it from a gym leader
by getting rock smash from oreburgh gym leader
you have to get the hm strength then beat the gym leader in canalave city
The first gym in Pokemon diamond is in Oreburgh City
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
The 8th Gym Leader has electric type pokemon. Your best bet to defeat him is Ground or Rock pokemon. Electric types are not effective against ground types.
Roark, a Rock-type specializing Gym Leader, can be found in Oreburgh City (just like in Diamond and Pearl).
The 6th gym leader is in Canalave City.
you beat the gym leader
He is in the lighthouse
It is Candace, The Ice-Type Gym Leader.