Other notable sulfur allotropes include rhombic sulfur and monoclinic sulfur. Of the other discovered allotropes, carbon nanofoam is a ferromagnetic allotrope discovered in 1997. Tellurium has two allotropes, crystalline and amorphous. The four other allotropes of selenium are metastable. They exhibit structures analogous to various allotropes of carbon, including graphite, diamond, and nanotubes. Gaseous allotropes exist as well, such as dioxygen and ozone. The two most common allotropes of pure carbon are diamond and graphite. Phosphorus has several allotropes that exhibit strikingly diverse properties. Polonium is a radioactive element that exists in two metallic allotropes. Arsenic and antimony form several well known allotropes (yellow, grey, and black). Neptunium is found in at least three allotropes. These allotropes are known as the amorphous and the crystalline allotropes. The two most common allotropes are white phosphorus and red phosphorus. This multiplicity of allotropes is common among the actinides. The element normally exhibits six allotropes and four oxidation states.