Ah! Maybe that's what balthasarcraft meant, above.
Well, I don't know about that. I'm not entirely sure why they call it "diamond". True, rough diamonds are pretty dirty too, usually, I suppose. I guess it forms differently from other quartz, and more like diamonds? Dunno. Maybe if they cut a Herkimer Diamond, it would sparkle similarly to an actual Diamond? Never seen a cut one before, though.