Otsuki once found a stone called "方孔石:HOUKOU-SEKI;Stone with square hole" in SENDAI, which has diamond shape hole (maximum diameter 2 cm).
Later, he found that the hole was made by a "玄能石:GEN'NOH-ISHI" fallen from the stone.
He wondered that the GEN'NOH-ISHI is found only in the tertiary layer in Japan while HOUKOU-SEKI is found in Middle Century (I'm not sure for this translation).
Here is the image of HOUKOU-SEKI.
Here is a location where HOUKOU-SEKI is found (not SENDAI).