There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a miniature size hosta with slightly twisted, dark green leaves of thick substance. There appears to be some red dotting on the short petioles. This non-registered cultivar was originated by Sakata of Japan. 

Mikiko Lockwood in an article on The Hosta Library titled, A Little About Japanese Hosta Terms defines the term iwa as rock, Iwa Gibōshi or H. longipes.