|We must've been through hundreds of books already and we can't find him anywhere —|
Pyrites is the surname of a wizarding family which is most likely pure-blood. The family was never mentioned in the actual books, but appears on some earlier drafts which J. K. Rowling released on her official website.
Pyrite is a shiny yellow mineral that typically forms in clusters of cubic crystals. Due to its resemblance to gold, iron (III) pyrite is also known as fool's gold, which is probably why Rowling chose it as a name for a showy but sinister Death Eater.