If you want to incorporate some impurity - you can plug things in to modulate the Reflection softness, the Blinn eccentricity, or the Fresnel_Blend outer color (which basically controls maximum reflection level).

Here I used a Granite node to make some subtle spots. The blade on the right in the render has the Granite.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)