Also known as obsidian gas, it refers to the sharp boulder born on the sand around the cave site. Xu Shanji's "Instructions of the Son of Man: Shafa" says: "All dragons and tigers are elbow-shaped, and the dragon body has branches and feet, and the sand is left and right in front of the cave, and the gap between Shimonoseki Shuikou and the dragon body, but there are sharp stones and boulders, which are all obsidian stars."