Kubomi-ishi (Hollow Stones) (凹石)
Kubomi-ishi (hollow stones) refers to the stone tools that have a shallow hollow almost in the center of fist-sized round gravel and elliptic gravel. The hollow was the result of receiving a blow, or the one of rotary frictions; the former showed that the stones were used for breaking the walnut shell and manufacturing stone tools while the latter indicated that pestles for making a fire were pressed down on the stone.
Beehive stones were the stone tools with many hollows. Unlike kubomi-ishi, board-like stones are selected as materials in many cases; because hollows mainly resulted from rotary frictions, beehive stones are seen as making fire mortars. Beehive stones are also called Amadare-ishi (raindrops stones).