An alloy of copper and gold, usually in a ratio of about 24:1. When polished it looks almost the same as pure copper, but it is very difficult to keep polished. Instead it is valued for its deep tarnished color; depending on exactly how it is treated, it can be a dark blue-gray (Baldwin's Patina or salt and ammonia) or purple-gray (rokusho, or simply firescale) color. Because it can be corroded away by the oils on your skin, it is rarely used for jewelry, although it is used in some pieces as part of the Mokumé Gané (lit. 'burl-grain metal') patterns, in which metal is folded into swirls resembling woodgrain. The dark color works especially well in contrast with gold.
Shakudo is probably a Chinese creation, although it is possible that it was first used somewhere further to the west, and has been around at least since the Han Dynasty. It crossed over to Japan sometime in the 1400s, and is now best known for its use in Japanese metalworking, where it was traditionally used in decorative inlays on the hand guards (tsuba) of katana swords, menukis, yatate, and other fancy pretties for the wealthy.
The term shakudo is also sometimes used to refer more generally to damascened (inlayed) Japanese jewelry as a class.
Compare to shibuichi, a copper and silver alloy.
Melting point: 1900°F. Annealing range: 1150-1350°F.
Thanks to liveforever for the kanji.