Osmium

Osmium is a silvery metal – the densest of all known elements. It is almost twice as heavy as lead, and has a high melting point.

In combination with other elements, it produces extremely hard alloys that are used for fountain pen tips and pivots for various instruments. Also, osmium is alloyed with platinum in the manufacture of cardiac pacemakers and replacement heart valves.

One compound – osmium tetroxide – is used to detect fingerprints. This volatile compound is extremely poisonous and can lead to temporary blindness, but its irritating odour deters one from extensive exposure.