Stones getting shaped by the ocean, very beautiful

Isn't it from underwater volcanoes or something? The sand gets super-heated?

It's actually from when people throw bottles in the water and the broken glass is smoothed from the motion of the water! Though the volcanoes thing sounds really cool.

They're used for earrings and necklaces.

Obsidian, the volcanic glass you're thinking of isn't from the sand being heated, it's the lava itself and its black.

They’re smooth but have a slightly rough texture to them