There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a large size hosta with green leaves and creamy white marginal variegation. This non-registered cultivar was originated by Kosasa of 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 nishiki as brocade (rich-colored woven fabric) i.e. something colorful and beautiful.