Finding black streaks or splotches in the sand while walking along a Michigan beach may have you wondering if you've stumbled across some sort of pollution, but it's actually a completely natural phenomenon in most cases.

Michigan beaches often are streaked with an iron-based mineral called magnetite, the National Park Service pointed out this week in a Facebook post.

"Have you ever been walking the beaches at Sleeping Bear Dunes National Lakeshore and noticed patches of dark sand? Congratulations! You have stumbled upon some magnetite," the park service said in the post. "Sand here of course contains magnetite, usually accompanied by hematite — a mineral that gives it its signature color, but (the sand is) primarily made of quartz! Of quartz it is."

The dark-colored magnetite can be

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