Element (Hg), Atomic No. 80, density 13.55 kg/litre, M.Pt -38.4°C

A liquid at room temperature! Hardly suitable for coins, but would form solid alloys or amalgams (such as used in tooth fillings).

Source: http://www.tclayton.demon.co.uk/metal.html

Mercury is a metal that has been of great alchemical importance in ancient times. In ancient China there is evidence that mercury was used by the latter half of the first millennium BC mercury while mercury metal is reported from Hellenistic Greece. Mercury is a volatile metal which is easily produced by heating cinnabar followed by downward distillation of the mercury vapour. Some of the earliest literary references to the use of mercury distillation comes from Indian treatises such as the Arthashastra of Kautilya dating from the late first millennium BC onwards. Some evidence for mercury distillation is reported from the ancient Roman world.

In India, vermilion or cinnabar i.e. mercuric sulphide has had great ritual significance, typically having been used to make the red bindi or dot on the forehead usually associated with Hinduism. Ingeniously in ancient Chinese tombs cinnabar was used successfully as a preservative to keep fine silks intact. Mercury was also at the heart many alchemical transmutation experiments in the Middle Ages in Europe as well as in Indian alchemical texts which were precursors to the development of chemistry.