Why is gold yellow? White, rose and green gold

Why is gold yellow? #

Gold absorbs part of the blue light and reflects the rest, which our eyes see as yellow. This is explained by how the electrons move in such a heavy atom. In most metals the reflected light is silvery, which makes gold and copper exceptions.

Why does gold not tarnish? #

Gold is a noble metal: it reacts very little with oxygen and with most products, which is why it stays bright for centuries. See gold.

How the colors are made #

Pure gold (24 karat) is very soft, so in jewelry it is alloyed with other metals, and that mix gives the color. See jewelry alloys and the color of metals.

• Yellow gold: gold with silver and copper in balanced proportions.
• Rose gold: more copper; the more copper, the redder.
• White gold: gold with white metals such as palladium, and sometimes silver or nickel. It is usually rhodium-plated for a brighter white, which wears off over time.
• Green gold: gold with silver, with a soft greenish tone.

All have the same karat: an 18-karat white gold contains 75% gold, just like a yellow or rose one. See 14 or 18 karat gold.

Are there other colors of gold? #

Colors such as blue or black are achieved with surface treatments, not with the alloy. See coloring of metals.