Phosphorus

Hennig Brand, a Hamburg alchemist hunting the philosopher's stone, boiled down some 5500 litres of urine in 1669 and was left with a waxy solid that glowed green in the dark and burst into flame in air — the first element discovered by a named person, and discovered for entirely the wrong reason. White phosphorus is that allotrope: it ignites spontaneously above about 30 °C and was the cause of phossy jaw in Victorian match factories, which is why safety matches use the far tamer red allotrope, striking it on the box rather than carrying it in the head. Life runs on phosphate: the backbone of DNA and RNA, the P in ATP, and the mineral of bone. There is no substitute and no synthetic route, so the world's fertiliser depends entirely on mined rock phosphate.