Volcanic!

The last volcanic activity in these parts must have been billions of years ago. As I explained here, boulders were carried here as glacier morene 30.000 years ago. As they are dug up or thrown aside by farmers after plowing, these are the only rocks to be found in this country. Farmers in these parts used them as pavement

Every now and then I take one with me for the same purpose.

Inspired by Appreciation's texture blip