 Four <unk> of antimony are known : a stable metallic form and three metastable forms ( explosive , black and yellow ) . Elemental antimony is a brittle , silver @-@ white <unk> metalloid . When slowly cooled , molten antimony <unk> in a trigonal cell , <unk> with the gray allotrope of arsenic . A rare explosive form of antimony can be formed from the electrolysis of antimony <unk> . When <unk> with a sharp implement , an <unk> reaction occurs and white fumes are given off as metallic antimony is formed ; when rubbed with a <unk> in a mortar , a strong detonation occurs . Black antimony is formed upon rapid cooling of vapor derived from metallic antimony . It has the same crystal structure as red phosphorus and black arsenic , it oxidizes in air and may ignite spontaneously . At 100 ° C , it gradually transforms into the stable form . The yellow allotrope of antimony is the most unstable . It has only been generated by oxidation of stibine ( <unk> ) at − 90 ° C. Above this temperature and in ambient light , this metastable allotrope transforms into the more stable black allotrope . 
