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 green leaves and yellow medial (center) variegation. Some of the lanceolate, pointed leaves twist to the side and are quite distorted. This non-registered cultivar was originated in 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 and the term shironakafu or shiro nakafu as white-center(ed).