Aluminium
It’s the most abundant metal in Earth’s crust, yet it entirely escaped our notice until 1825
You might say it was hidden in plain sight. Aluminium is a highly reactive metal, meaning it readily undergoes chemical reactions with other elements and compounds to form different substances. As a result, nearly all of the naturally occurring aluminium atoms on Earth ended up tucked away in the molecules of more than 270 different minerals, including gemstones like emeralds and rubies. While it’s actually 8.2 per cent of the Earth’s crust, making it the most common metal and third-most common element, behind oxygen and silicon, you would never know it’s there without investigating on a chemical level.
The search was on in the mid-1700s, when chemists began experimenting with alum, a class of abundant chemical compounds. Alum compounds, such as potassium aluminium sulphate, were well known, going back at least to the ancient Greeks and Romans, who used them as an astringent to close wounds and a mordant to bind dye to cloth. Early chemical investigation of alum suggested that the compound included an unknown metal.