Chromium vs Manganese

Chromium takes its name from the Greek word for colour, and it deserves it. Traces of chromium make rubies red and emeralds green. Its compounds gave us the yellow of school buses and Van Gogh's sunflowers, the green of old wallpaper, and the orange of the potassium dichromate in a school lab. Yet the metal itself is grey, hard, and almost boringly resistant to tarnish.

• The same element gives ruby its red and emerald its green: chromium atoms replacing aluminium in two different host crystals absorb light differently.
• Chromium is among the hardest pure metals and takes a high polish, which is why chrome plating is used both for decoration and for wear resistance.

Manganese is one of the unsung workhorses of the industrial world. About 90 per cent of it goes into steel, where it removes oxygen and sulfur from the molten metal and toughens the finished product. Without manganese, steelmaking as we know it would not work, and there is no good substitute.

• The oxygen released by every plant, alga and cyanobacterium comes from a water-splitting cluster containing four manganese atoms in the protein called photosystem II.
• The ocean floor holds vast fields of manganese nodules, which grow by only a few millimetres every million years.