Magnetite

Magnetite is a black, iron, magnetic sand that is common in most sands though in very small quantities. It concentrates in various places because it is heavier than the rest of the sand grains and results in black sand patches along the beach, where energy was a little lower because of currents, bars, rips or structural things like geologic strata that is harder than sand. The magnetite accumulates over time, , left behind on the beach as the lighter silica sand is washed out into the Gulf with the eroding beach.