Black stone; Dry landscape

Quick answer：

Black stone, also known as 'Heishi' in Chinese, refers to a type of smooth, round stone often used in Japanese Zen gardens known as 'Karesansui' or 'Dry landscape'. Karesansui is a form of Japanese rock garden design that uses stones and raked sand or gravel to represent a natural landscape, typically mountains and water. The black stone adds a sense of depth and contrast to the garden's composition.