Growing copper nano™ crystals

By accident came across process of growing metallic copper crystals.

The technology is the following: put some CuSO4 in a tube, then some NaCl on the top, and add some concentrated solution of NaCl (using distilled water). Then put some iron inside - and wait few days/weeks. Initially, I've tried that with nickel - but it didn't worked. Obviously, tube must be sealed - otherwise solution would just evaporate and everything would be contaminated by NaCl crystals.

I've decided to try 3 variants: without extra salt, with 0.5 thickness of salt, and 1.5.

~1 week later we see the following: gray screw - without extra slat, black - with 1.5 salt, and the most beautiful - with 0.5:

Let's take a look under microscope:

I guess, it is possible to grow larger crystals if one would try salt thickness around 0.5 and waiting for a longer time.