Shakuō (赤銅) is a billon of gold and copper (typically 4% Gold, 96% Copper), mostly designed for its beautiful dark blue-purple patina. It was historically used in Japan to decorate katana fittings such as tsuba and yatate. When it was introduced to the West in the mid-19th century, it was thought to be previously unknown outside Asia, but recent studies have suggested close similarities to certain decorative alloys used in ancient Egypt, Greece, and Rome.. Modern jewelry artisans have revived the use of shakudō as a striking design element, especially for the technique of mokume-gane.