When I was younger I went to a beach in southeast Queensland. This beach has streaks of black sand and I was convinced that it was some terrible environmental disaster that was sure to be toxic or poisonous or worse (not sure what could be worse now, but at the time it was terrible). I had no one to ask at the time and the ‘toxic black sands’ from that day were quietly forgotten.
I have since learned that this was not an environmental disaster, but instead an interesting effect of mineral density. Black sands are simply black minerals (as opposed to white/yellow/red quartz sands) that appear as streaks because they settle at approximately the same position in the water column. Australia is a significant exporter of mineral sands (though not mined at tourist beaches for obvious reasons) which is perhaps not surprising as these deposits exist as ancient sand dunes and Australia is both ancient and sandy.