The letter Z was borrowed into the Latin alphabet in order to transcribe Greek loanwords, along with Y.
Presumably, educated Latin-speakers pronounced it like its source, Greek zeta. However, Greek zeta had various different pronunciations across time periods and dialects.
What can we say about the Latin pronunciation of Z in loanwords? Do any ancient grammarians comment on this, for example? Or can transcriptions of Greek words into other languages confirm its pronunciation at that time?