I’ve been to black sand beaches in Hawaii and Iceland. Black sand beaches are rare and found in volcanic formation only, and aquatic black sand habitats are even rarer. Larva rock range from black to red and go well with black sand which is derived naturally from parent larva rock. I use larva rock in my white sand substrate though because larva rock is light in weight, safe and easy to stack up, and provides porous surfaces for attachment of Java fern and Anubias. But my goal is to have vegetation completely hide the red rock so it doesnt stand out unmatching the white sand.