Four allotropes of antimony are known : a stable metallic form and three metastable forms ( explosive , black and yellow ) . Elemental antimony is a brittle , silver @-@ white shiny metalloid . When slowly cooled , molten antimony crystallizes in a trigonal cell , isomorphic with the gray allotrope of arsenic . A rare explosive form of antimony can be formed from the electrolysis of antimony trichloride . When scratched with a sharp implement , an exothermic reaction occur and white fumes are given off as metallic antimony is formed ; when rubbed with a pestle in a mortar , a strong detonation occur . Black antimony is formed upon rapid cooling of vapor derived from metallic antimony . It have the same crystal structure as red phosphorus and black arsenic , it oxidize in air and may ignite spontaneously . At 100 ° C , it gradually transform into the stable form . The yellow allotrope of antimony is the most unstable . It have only been generated by oxidation of stibine ( SbH3 ) at − 90 ° C. Above this temperature and in ambient light , this metastable allotrope transforms into the more stable black allotrope .