The Collaborative International Dictionary of English v.0.48:
Diamond anniversary \Diamond anniversary\, Diamond jubilee \Diamond jubilee\, etc. an anniversary celebrated upon the completion of sixty, or, according to some, seventy-five, years from the beginning of the event commemorated. [Webster 1913 Suppl.]WordNet (r) 3.0 (2006):
diamond jubilee n 1: an anniversary celebrating the passage of 60 years