There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a miniature to small size hosta with green leaves and irregular yellow medial (center) variegation. The amount of yellow varies from leaf to leaf with some being almost all yellow while others on the same plant are all green. 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.