What is Kronos' scythe made out of?

First off, Kronos didn't wield a scythe, he carried a sickle, which is a whole different thing.

Second, it was made of nothing, because it doesn't exist.

If you insist on being that specific, though, it's reasonable to say that to the most ancient Greeks, it would have been made of stone, and later bronze, which was the hardest metal they knew how to forge into weapons. Later, it would have been envisioned as iron or steel, based on the technology of the time.