Im sorry, but I do not have access to the grading or ranking system of slate materials in Norway. However, I can provide general information about Norways Finna Slate and its uses. Finna Slate is a high-quality natural slate material found in southwestern Norway. It has been used for centuries as a roofing and cladding material due to its durability, low absorption rate, and unique blue-gray color. It is also popular in interior design for its aesthetic qualities.