Most club faces are steel. Steel transfers energy efficiently — but there's a ceiling on how hard it can be without becoming brittle.
DID's Z Face uses zirconia ceramic, with a Vickers hardness of 1,100–1,300 HV. For reference, steel sits around 200 HV. Diamond is 10,000 HV. We're not claiming we put diamond in a club — we're saying we moved significantly up that scale in a way that translates to measurable ball speed.
Harder face. More energy transfer. The physics works.