text: Classical examples of polymorphism are the pair of minerals calcite and aragonite, both forms of calcium carbonate. Allotropy is the term used for elements, for example diamond versus graphite, and in metallurgy.
  β-HgS precipitates as a black solid when Hg(II) salts are treated with HS.  With gentle heating of the slurry, the black polymorph converts to the red form.