The contents of the codices were written in Coptic, though the works were mostly (all?) translations from Greek. Most famous of these works must be the Gospel of Thomas, of which the Nag Hammadi codices contain the only complete copy. Incomplete versions were recognized in the 19th century from fragmentary manuscript finds and quotation in other sources.
The date of the lost Greek originals behind the Coptic translations is controverted, but the manuscripts themselves are from the 3rd and 4th centuries.
Robinson, James M., ed. The Nag Hammadi Library in English ISBN: 0060669357