pyrite
Concise Oxford English Dictionary © 2008 Oxford University Press:
pyrites /pʌɪˈrʌɪtiːz/ (also iron pyrites; Mineralogy pyrite)
▶noun a shiny yellow mineral consisting of iron disulphide, occurring as intersecting cubic crystals.
– derivatives
pyritic adjective
pyritize (or pyritise) verb.
pyritic adjective
pyritize (or pyritise) verb.
– origin ME (denoting a mineral used for kindling fire): via L. from Gk puritēs ‘of fire’, from pur ‘fire’.
'pyrite' also found in these Oxford entries:

