Lake Michigan dune sand is mostly quartz, but other minerals such as feldspar, magnetite, hornblende, garnet and calcite are also present. The magnetite and other dark minerals like hornblende sometimes collects in dark patches or layers on the beach and dune surfaces. On the beach, water from waves often causes the dark minerals to accumulate. On the dunes, the dark minerals accumulate as wind or gravity moves sand around.

Some people mistake the dark shiny patches for pollution. The magnetite grains are magnetic. Try wiping a magnet across the surface of the sand to see what will happen.