1) Precious metal alloy 
Shakudō (赤銅) is a billon of gold and copper (typically 4–10% gold, 96–90% copper) which can be treated to form an indigo/black patina resembling lacquer. Unpatinated shakudō visually resembles bronze; the dark color is induced by applying and heating rokushō, a special patination formula. Shakudō was historically used in Japan to const...
• (n.) An alloy of copper, invented by the Japanese, having a very dark blue color approaching black.
An ornamental metalworking technique which produces a decorative black layer.
Shak'u·do' noun [ Jap.] An alloy of copper, invented by the Japanese, having a very dark blue color approaching black. 
Copper and gold alloy used for sword fittings
Copper and gold alloy used for sword fittings.