There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a small size hosta with narrow, dark green leaves and creamy yellow streaked variegation. This non-registered cultivar was originated in Japan by Yokoe. 

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.