There is an image of this cultivar on the Hosta Library. From the photo, it appears to be a medium to large size hosta with dark green leaves and creamy white streaked variegation. This is a form of the species, H. longipes, which is a non-registered cultivar from Japan by Hagi. 

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.