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
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.