Antimony is a chemical element with symbol Sb ( from Latin : stibium ) and atomic number 51 . A lustrous gray metalloid , it is found in nature mainly as the sulfide mineral stibnite ( Sb2S3 ) . Antimony compounds have been known since ancient times and was used for cosmetics ; metallic antimony was also known , but it was erroneously identified as lead upon its discovery . In the West , it was first isolated by Vannoccio Biringuccio and described in 1540 , although in primitive cultures its powder have been used to cure eye ailments , as also for eye shadow , since time immemorial , and is often referred to by its Arabic name , kohl .