Specular material bro. Index is the one you need to tweak most often.. though, default (1.45)
should be pretty darn close... 1.3 is what you use for water if I remember correctly. Just play
with the settings.. you will get there.
And read manual during test renders
Edit: oops, I just re-read your post.. Mix Material node. Plug a specular and a diffuse into it.
For Diffuse node, plug an image into diffuse pin and an alpha (b/w 24bit image of what you
want and what you don't) into opacity (also in diffuse node). You should not even need to tweak
the
amount for your mix material node.
I just did a quick test of this for the hell of it.. 1.1 to 1.2 for index seemed better.. and add
a little roughness. Lookin' pretty good in under two minutes...