I recently purchased a version of the magsafe phone case that has a magnetic ring in it to attach accessories. While playing tennis, I put my phone on the clay court and when I picked it up, I was surprised to see a ring of clay stuck to the phone, as shown in the photo. The magnet field had attracted a material in the clay. Most clay courts, like those used in the French open, are red colored and made of crushed brick, a mixture of clay and sand with no magnetic properties. But, it turns out that green clay courts, which are found almost exclusively in the Southern US, are made of metabasalt which contains a high concentration of iron oxide, which is magnetic.