Black Basalt Gravel 20mm is a hard-wearing volcanic rock that shifts from a stylish smoky grey to a rich jet-black when wet, enhancing its visual impact. This gravel, primarily composed of angular particles, naturally interlocks, making it extremely stable underfoot and suitable for driveways and paths. It ranges in actual size from 8mm to 25mm, ideally laid at a depth of 3-4 cm to achieve optimal coverage and stability.