One of the most frequently asked question by guests of the YODOKO Guest House is what the stone used for the interior and exterior works is. This stone is called "oyaishi," and is quarried at Oya-machi in Utsunomiya City, Tochigi Prefecture. Oyaishi was also used for the Old Imperial Hotel, a representative and renowned work by Wright, and it is said that oyaishi is what characterizes Wright Architecture in Japan. In truth, it was not oyaishi, but hachi-no-su ishi from Shimane Prefecture that had been initially considered for the building. The amount of hachi-no-su ishi produced, however, was too low, and they had to give up on the idea. Why Wright decided on oyaishi is explained here.

At the YODOKO Guest House, oyaishi is extensively used throughout, including exteriors, interior pillars, stairs, etc. Furthermore, it is not used just as a piece of stone, but rather sophisticated designs of local plants are carved as motifs. It can be said, therefore, that by taking into consideration that Wright would have wished to display his original geometric decorative designing, he decided on oyaishi because it was soft and easy to process. The texture of the uneven surface creates an obviously easy feeling, and the warm coloration of brown matches well with the colors of the exterior walls, etc. Since oyaishi is tuff which was formed by volcanic activity approximately 20,000,000 years ago, pieces of wood and seashells sometimes can be found in it.