Just as for fossils, there are no laws specific to geology in France. Although, we know that we can not search in a geological natural reserve.
However, the article 552 of the Civil Code on the ownership of real estate and movable property requires us to have the agreement of the owner of the land to detach a stone from the basement (such as crystals on a rock), move or recover a stone on the ground.
A geological stone attached to the basement is considered a real estate (immovable objects) and a stone detached or present on the ground is seen by the law as a movable property (such as a furniture).
If the land belongs to the state, it must be agreed by the town hall, and see if it is not necessary also to get an additional agreement from the prefecture.
This law dating from 1804 is quite horrible, because it means that it is forbidden to pick up a pebble on the beach, to take a stick and throw it at our dog, to pick a flower, a succulent plant, thyme, rosemary, wild fruits, to pick up a lost coin, … without having the agreement of the owner of the land on which we are. We can not even pee on a soil that does not belong to us, because it is damaging the real estate of others. In other words, everyone is in the illegality.