Element 24 in our International Year of the Periodic Table elements series is chromium. It’s the element that gives rubies their colour and is also responsible for the characteristic yellow colour of school buses.

The colour of many gemstones is due to the presence of impurities. In the case of rubies, which are composed of aluminium oxide, it’s chromium impurities in the structure which give it its colour. Chromium impurities can also cause contribute to colour in alexandrites, jade and spinels.

Chromium is most commonly known from its use in chrome-plating. Electroplating metals with a thin layer of chromium is used to decorate them and protect them from corrosion. It’s commonly used on tools and car parts.

Chromium also sees use in some pigments. In particular, the traditional yellow colour of school buses is from the chrome yellow pigment.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.