Mokume Gane:

Timothy Grannis has perfected the forging of Mokume Gane, a time-honored Japanese metalworking technique used in many of his designs. Mokume Gane (a Japanese word meaning wood-grain metal) is a process that forges together different metals. In the case of Timothy's jewelry, finely layered colored golds are carved and forged into a wood-grain pattern in 14k, 18k, and 22k.

The making of Mokume Gane is centuries old and dates back to Japan where samurai swordmakers found that by layering a harder high carbon steel (which holds an edge but is brittle) with a tougher low carbon steel (flexible but doesn't hold an edge) they could perfect a katanah blade that was flexible and stayed sharp. The patterns they found they could achieve in their blades opened up Mokume Gane as an art form.