Selenium shows borderline metalloid or nonmetal behaviour. Elemental antimony is a brittle, silver-white shiny metalloid. Sometimes tellurium is not regarded as a metalloid. The nonmetal selenium (Se) is sometimes instead classified as a metalloid, particularly in environmental chemistry. Germanium is usually considered to be a metalloid rather than a metal. At standard temperature and pressure, boron is a trivalent metalloid that has several different allotropes. Being a metalloid, most of its chemistry is nonmetallic in nature. It is the most important source for the metalloid antimony. Selenium is commonly described as a metalloid in the environmental chemistry literature. It is not certain whether polonium is a metal or a metalloid. A heavy metal is any relatively dense metal or metalloid. Some sources refer to polonium as a metalloid, although it has some metallic properties. The term "metalloid" originally referred to nonmetals. There is disagreement as to whether polonium is a post-transition metal or metalloid. Classifying aluminium as a metalloid has been disputed given its many metallic properties.