Mercury(II) sulfide is a red or black solid. It is found in two forms; cinnabar (alpha form) and metacinnabar (beta form). The alpha form is more common. The beta form is made when a mercury(II) salt like mercury(II) chloride reacts with hydrogen sulfide. It reacts with only the strongest acids to make hydrogen sulfide and a mercury(II) salt again. It does not dissolve in water. It breaks down to sulfur dioxide and mercury metal when heated.