Do you ever feel some invisible force drawing you up the trails to the boulder fields along the Princeton Ridge, whether at Herrontown Wood or Witherspoon Woods? It just might be their inherent magnetism interacting with an iPhone, binoculars, or, if you've been to the shop, one or another artificial joints. I had no idea until a neighbor approached me at the annual Memorial Day picnic at Potts Park, introduced himself as Jon Johnson, and then proceeded to tell me all sorts of interesting things about a part of the woods I thought I knew pretty well.
If you take a strong magnet along (keep it away from any electronics like phones or cameras), you can test the gravel in various tributaries to see if there's magnetite in the rocks. It can vary from 2% up to 20%.
Some boulders will be magnetic, too, but only portions thereof.