Kamuikotan Stone

The stones come from the Kamuikotan gorge, where the Ishikari River cuts through metamorphic rocks west of central Asahikawa. The gorge is the type locality of the Kamuikotan metamorphic rocks, formed deep in an accretionary complex by low-temperature, high-pressure metamorphism during westward subduction of oceanic crust that continued into the Late Cretaceous. According to the 1955 survey, especially hardened parts of the rock escaped weathering, remained as rounded blocks in the soil and were further smoothed after falling into the river; blocks of one to several tonnes were removed in their natural shape as garden stones.

Kamuikotan stone looks similar to serpentinite but is very hard. It is valued for a smooth, lustrous skin even without polishing, which earned it the popular nickname abura-ishi ("oil stone"); because the rock is so hard, strongly varied shapes are uncommon, and mountain, rock and island shapes are typical.

Kamuikotan is an Ainu place name; the Hokkaido geosite description explains it as "the place where the demon god lives". The rapids were a dangerous passage for boats and later for railways and roads. Potholes worn into the riverbed were traditionally said to be the demon god's footprints, a legend recorded by the explorer Matsuura Takeshiro in his Ishikari diary; the potholes are an Asahikawa city natural monument and the gorge's metamorphic rocks are among Japan's 100 selected geological sites. Kamuikotan stone is known throughout Japan as a suiseki and garden stone.