Why your gold ring turns green, and it’s not really the gold’s fault

Scientists at Tulane University have finally explained why pure gold never tarnishes, and the discovery also clears up why the ring or necklace in your jewellery box still turns dull, black or green.

The answer is that pure 24-karat gold really doesn’t tarnish. It’s the metals mixed into it that cause the trouble. Because 24-karat gold is too soft for everyday wear, jewellers alloy it with copper, silver, zinc or nickel to make it stronger. Copper darkens and turns brown or green when it meets sulphur and sweat. Silver reacts with sulphur compounds in the air to form a dark grey or black layer. Zinc and nickel can oxidise over time and leave green marks on acidic skin.

How much of this happens depends on karat rating. An 18-karat piece is 75% pure gold, 14-karat is 58.3% and 10-karat is just 41.7%, meaning lower-karat jewellery contains more alloy metal and tarnishes faster when exposed to moisture, cleaners and everyday air.

A person’s own body chemistry plays a role too. People with more acidic sweat often see their rings and bracelets discolour within months, while ammonia-based cleaners, lotions, perfumes and hairsprays all leave residues that react with the alloy metals. White gold has a separate issue: its bright rhodium coating simply wears off with use, exposing the yellowish alloy underneath.

The underlying reason pure gold resists all this comes down to its atoms. Researchers found that surface atoms on gold rearrange themselves into patterns that block oxygen from reacting with the metal, slowing tarnishing by up to a trillion times.

Image: Wikimedia Commons/by Stevebidmead