Kediet ej Jill mountain in the Sahara desert, Mauritania. It seems bluish because of the high concentration of magnetite, an iron ore and a natural magnet. Because it is magnetic, the mountain disrupts navigational compasses.
He whispered in my ear about the mountain: a huge solid block of iron in the gravel not far from here. A massive magnetic chunk whose force fritzed all signals, bankcards, computers.