https://www.allegorithmic.com/blog/path-substance-sourceLooks they've got a pretty decent library of materials built up allready. I'm excited to see where this goes.
I've just downloaded a few of the free materials, and unfortunately it looks like it is only sbsar, so you can't pick apart the material graph.
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it's a shame that's it's only .sbsar - would have been quite educational dissecting some of those substances, although, I imagine quite a few of them will be a maze to go through
http://polycount.com/discussion/179334/substance-source-master-thread