I've used years ago an inductive proximity sensor for a test, and I remember that it sensed aluminium and copper way better than iron.

Just found this doc. http://www.rselectronics.com/SEO/Selecting_Prox.pdf

Yes - inductive sensors work on the same principles as a comercially avaliable metal detector. They have an LC circuit which oscillates in a particular frequencey range, when a conductive material is near by, eddy currents form in the material which then produce their own magnetic field which then dampens the oscillation of the sensor.