Rocks are resource objects placed randomly on meadows. They seem to occur more often on the meadows in the interior of the map, and much less in the smaller, outer meadows. Hammers are required to mine rocks, and you will get 0 - 3 gold from each rock.
Like with other resources, minions that can loot them will move to them when close. It takes 4 - 10 hits to get gold from a rock, if any. There is currently no perk which makes rocks give more gold.
Rocks, like other resources, respawn when their meadows are rained on.