Gokoiwa (Light Spectrum Rock)
Gokoiwa is a massive rock 10 meters in diameter that sits on the riverbed in the upper reaches of the Ryuokyo Valley. It is made of green tuff, formed when hot ash in the volcanic eruption fused various rocks together. The hardness of the rock is uneven, which causes different rates of erosion, leaving a hole in the middle of the rock. The name is said to derive from a legend that the cave refracted light into five colors, though this phenomenon is not visible today.