Non Metallic Metal
Non Metallic Metal (NMM) is a relatively new technique in miniature painting, but it is actually an old technique, which dates back to old paintings and drawings.
You didn't have metallic paint to splash on a canvas back then and even if you had a big solid square of metallic paint would not look anything like metal anyway.
The aim of painting NMM is to simulate a reflection of the horizon in the plate of the armour, it sounds easy and it is if you know what to do.
What you need to be able to do is some fine blending, the basic idea is to paint two gradients on the plate: One goes from the top of the plate to the middle. It starts dark and goes into white in the middle. The second gradient goes from the middle to the bottom of the plate and goes from being dark at the middle to white ate the bottom.
That will give you a very basic NMM, but if you want to make it look even more real you should make the horizon a bit curvy.
The best way to do this is paint the top gradient first not really thinking about the curvy line of the horizon. The paint the bottom gradient on top, marking and defining the horizon with the dark colour.
There is one last thing to learn when painting NMM on miniatures:
On paintings everything would use the two gradient technique, but it doesn't work on a miniature simply because the plates can be too small to possibly give a good result.
It is a matter of choice when to use the two gradients and when not to. However when you don't use the two-gradient technique, you should do one gradient going from dark at the top to white at the bottom.