There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a medium size hosta with dark green leaves and dark purplish flower scapes. This non-registered cultivar was originated in Japan and introduced by Dr. Ralph (Herb) Benedict of Michigan. 

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.