Black beach sand originates from broken down what?

Black sand is made of broken down fragments of volcanic rock. When a volcano erupts, lava pours out, cools down, and creates black rocks. The waves erode and break down this rock causing for black sand to be created.

Sand grains are formed when rock or other hard material is broken down by waves, a process that can take thousands of years. Beaches with pebbles or course sand are very young while beaches with fine sand are older. The color of sand grains comes from the original material that formed the sand.