This is the famous Japanese traditional color "紺青" (konjō) — a deep, elegant blue with purple tones. It also has an ancient name, "金青" (kinsei, "gold blue"), so called because its raw material, the precious mineral azurite, was considered worthy of the character "金" (gold). According to the ancient Chinese Mustard Seed Garden Manual of Painting, when azurite (石青) is ground and washed in water, three grades result — first blue, second blue, and third blue — with the deepest third blue corresponding to konjō.