Yes, the peak of Mt Coronet (also known as Spear Pillar) can be accessed via the Hearthome City-side entrance (Route 208). It can also be accessed from Route 207, near Oreburgh City.
You should note, though, that Spear Pillar cannot be accessed from any entrance until you encounter Cyrus at the Galactic Veilstone Building and release Uxie, Azelf and Mesprit.
The Mt. Coronet summit.
You can find it in Mt. Coronet (B1F)
Go to Hearthrome, West, follow path, surf (there is a rectangle of water near the entrance), Keep on going,you'll need waterfall to climb the waterfall ,then you will reach the peak. At the top is spear pillar where you can catch Dialga.
After you get the HM and earn the Fantina in the Hearthome City Gym
In the Hospital. Because she has diarhea.
You need to work your way up using strength rock climb and rock smash from the entrance to the left of hearthome
You have to go to mount coronet (entrance nearby hearthome city) and seek your way trough the cave. If you don't find it; there are vids on youtube.
You need to be able to use Rock Climb first (Rock Climb from South East entrance from Hearthome)
You have to enter Mt. Coronet from the Hearthome City entrance (aka on the right side of the Mt ) and then go to the upper right hand corner. The rest of the way is pretty much set.
To the right of Mt. coronet
go in by Hearthome city
Mt. Coronet and Trophy Garden (the garden behind the mansion near hearthome city)
You receive it from a hiker at the Mount Coronet entrance.
talk to the hiker at mt coronet range entrance
Mt coronet is near: Eterna, Oreburgh(East), Hearthome & Celestic(West) kinda in the middle of sinnoh
mount moon isn't in Pokemon diamond. it would be in FR and LGMount moon is located in Kanto, not Sinnoh where Diamond is set. You may be thinking of Mt. Coronet which is accessible from Orebrough or Hearthome city.
you go through mt coronet from the hearthrome or orebourgh city entrance and up a load of stairs.