During a recent trip to the Portuguese island of Madeira, I found a couple of interesting rocks on a beach. Both the rocks pictured in this question contain the same black mineral. I first guessed this was tourmaline but the luster isn’t quite right as it’s more metallic than vitreous or dull. Madeira is a volcanic island so I’m assuming that the the rocks are igneous.
Info: Black colour, white streak, small pieces attracted to a strong magnet, metallic luster, can be scratched by a steel knife (although takes a bit of effort).