Rhino material to GH material?

Dim mat As New rhino.DocObjects.Material

(...)

Dim ghmat As New grasshopper.Kernel.Types.GH_Material(mat.Id)

print( ghmat.IsValidWhyNot() ) ---> Material is a null reference.

How it is done to convert from custom rhino.material (done in GH) to Gh_material (for use with preview comp.)? and vice versa?

I found no references in the forum. I want to create custom materials linked between Rhino and GH.

Thank you very much!