Red Gold - Black Gold

For the colloquial term "black gold", see crude oil.

Black gold is a type of gold used in jewelry. Black-colored gold can be produced by various methods:

• Electroplating, using black rhodium or ruthenium. Solutions that contain ruthenium give a slightly harder black coating than those that contain rhodium.
• Patination by applying sulfur- and oxygen-containing compounds.
• Plasma-assisted chemical-vapor deposition process involving amorphous carbon
• Controlled oxidation of gold containing chromium or cobalt (e.g. 75% gold, 25% cobalt).

A range of colors from brown to black can be achieved on copper-rich alloys by treatment with potassium sulfide.

Cobalt-containing alloys, e.g. 75% gold with 25% cobalt, form a black oxide layer with heat treatment at 700 - 950 °C. Copper, iron and titanium can be also used for such effect. Gold-cobalt-chromium alloy (75% gold, 15% cobalt, 10% chromium) yields a surface oxide that's olive-tinted because of the chromium(III) oxide content, is about five times thinner than Au-Co and has significantly better wear resistance. The gold-cobalt alloy consists of gold-rich (about 94% Au) and cobalt-rich (about 90% Co) phases; the cobalt-rich phase grains are capable of oxide-layer formation on their surface.

More recently, a laser technique has been developed that renders the surface of metals deep black. A femtosecond laser pulse deforms the surface of the metal, forming nanostructures. The immensely increased surface area can absorb virtually all the light that falls on it, thus rendering it deep black.