Red mercury

Red mercury is a term used for a number of substances that are supposed to exist (or that could possibly exist). So far, no such substance has been found. The chemical element mercury most often occurs in a mixture with Sulphur, as HgS. That mixture is known as Cinnabar, and reddish in color. In pure form, mercury is silver in color.

The following assumptions about red mercury have been proved as false:

It could be part of a fraud, The title Red would refer to the former Communist Europe. The name was made during Cold War.
It could be a codename for very pure Uranium or Plutonium.
It could be a very potent conventional explosive. This would make it possible to build small hydrogen bombs.

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Red Mercury Media

Crystals of mercury(II) sulfide and several other mercury compounds are deeply colored red, but have no publicly known use in nuclear weapons.