I just noticed that objects with metal materials have tiny black speckles on their perimeter. I don’t think it was always like this, but I’m not 100% sure about that. At first I thought this was all materials, but, after some experimenting, it seems to affect ONLY metal materials. Interestingly, I can make a custom material that seems to mimic the metal material (at least in render mode) and the custom material does NOT have these speckles. (As a side note, one model is actually at the world origin…so,no, these are not far from the world origin)
I’m not sure this would be considered a bug/issue or not, and, even if it is, it might be super-low priority (understandable…especially since there seems to be a work-around with custom materials). I have a handful of metal materials in my kit, and I just wonder if I should re-make them as custom materials (I think this will be a trivial task) on my next release or just leave them be. [edit: I see it’s not ideal, though, because the material thumbnails are not representative of the metalic-ness of the material, unforturnately, so hopefully this can be fixed…one day 
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Below is an image (and file), showing the speckles on the edges of the metal material and also the settings for the custom material that does not have speckles.
black speckle example.3dm (4.6 MB)
webdunce_sys_info.txt (2.7 KB)
[another edit]
I added fillets to the model.That seems to make it worse. Personally, I use screen grabs of render mode for youtube video thumbnails. I feel certain it was not always this way.
[edit again]
It doesn’t happen in Rhino7
[yet another edit]
I can get the custom material preview icon to look metalic by setting the main color to black.
(it is the reflection color that gives the object its color)
