Your 14-karat necklace tarnishes 3 times faster than 18-karat, here’s why

New research explaining why pure gold never tarnishes has also shed light on a question many jewellery owners ask: why does one gold piece stay shiny for years while another discolours within months?

The answer lies in karat rating. An 18-karat piece is 75% pure gold, leaving 25% for alloy metals, while 14-karat gold contains 58.3% gold and 10-karat contains just 41.7%. Because lower-karat jewellery contains more copper, silver or zinc, it tarnishes much faster when exposed to moisture, household cleaners and everyday air.

Each alloy metal reacts differently. Copper, used to create the warm colour of rose gold, darkens and turns brown or green when it meets sulphur and sweat. Silver, added to balance yellow tones, reacts with sulphur compounds in the air to form a dark grey or black layer. Zinc and nickel, used to harden white and yellow gold, can oxidise over time and leave green marks on acidic skin.

Body chemistry matters too. People with more acidic sweat often notice their rings and bracelets tarnishing within months, and everyday products like ammonia-based cleaners, lotions and perfumes leave residues that speed up the reaction. Swimming pools and hot tubs pose an even bigger risk, as heat and chlorine weaken the molecular bonds in lower-karat gold over time.

The research, from Tulane University and published in Physical Review Letters, found that pure gold resists all this because its surface atoms rearrange themselves into patterns that block oxygen, slowing tarnishing by up to a trillion times, a shield lower-karat alloys simply don’t have.

Image: Wikimedia Commons/by Stevebidmead