This IS a rock --

It seems even prettier sitting on my desk, the reds and greens more banded looking than diffuse.

It is -- a dolomite / quartz (chert) blend found along the Aliakmon.

Real rocks are better than fake rocks.
Wild rocks are best of all! 

This is not a rock, either.

What is it?

A bit of concrete.

Keep this on file somewhere to compare to a real rock in case you get confused. 

This is not a rock --

What is it?

Give up?

It's what's left of a pop bottle that I found in the ashes of a campfire at the top of Mavrovouni.

So -- the answer is, glass.

Do not mistake this for rock.