Special Mineral: Molecular Arrangement influenced by Geomagnetism

[Proteus]

I once read about a certain mineral deposit, I believe somewhere in France, the molecular grid of which was influenced by the earth's magnetic fields at the time the mineral was formed. Because of this it was useful in paleomagnetism, as one could know from the age of the mineral where the earth's magnetic north was at that time. Unfortunately, I've forgotten how these deposits were called, and how the mineral in question was called.

 

Anyone know what I'm talking about, or, if not, does anyone know how to find this information on Google, ie, which keywords to use? I've tried a couple, but didn't find anything about this subject.