What gold alloy is used in the Nobel Prize medals?

Nobel Prize medals are made from 18-karat green gold, then plated with 24-karat gold.

Green gold is an alloy whose color comes mainly from gold combined with silver, with other metals present in smaller amounts. The 18-karat designation means the underlying alloy is 75 percent gold by mass; it does not mean the entire medal is solid 24-karat gold.

The medals awarded for the Nobel Prizes in Chemistry, Literature, Peace, Physics, and Physiology or Medicine have been produced in this general tradition since the prizes began in 1901. Modern medals are typically about 66 millimetres in diameter and weigh approximately 175 grams, although the Economics medal is slightly heavier.

A common mix-up is the medal’s outer appearance. The bright yellow surface comes from a layer of 24-karat gold, while the medal’s structural material is 18-karat green gold. Another historical detail is that medals made before 1980 were struck in 23-karat gold rather than the modern green-gold alloy.