The train from Amsterdam to leeuwarden / groningen stops in Zwolle. There you can catch the train to Emmen.
On the Emmen train stations you can take a bus or hire a bike to go to Borger (half an hour to an hour depending how fast you cycle).
At the Emmen tourist Office (VVV) located in front of the great ZOO you can get a cycling map that also marks all the hunnebedden and grave-hills and stuff.
The Zoo is also really worth a visit. They have 2 tiger-cups at the moment.

Happy stone-hunting