The dolerite is a vein rock, generated by the cooling of magma injected in faults a hundred metres deep in the underground.

It is composed of plagioclases, mafics minerals and iron titanium oxides.

Chemicals reactions cause the flaking of dolerite and produce balls surrounded with concentric scales like onion-skin and with a very marked oxidation (red-brown colour).

Place: Raguenez, Crozon.