Unfortunately, it's both. Or at least, it was.
Iirc (and I may not, but let's go with it for now), with Crafting (which the random weapon effects draw from), you used to be able to apply "Alchemical Silver" to any weapon - and so, yes, it is/was an effect.
You can
also find silver weapons - which are different, but have the same end-result.
The diff is/was that true Silver weapons are softer than steel that have had Alchemical Silver applies to them. So, actually, the two are not the same.
That, and I'm guessing it would be a pain to go back and change everything now.
(I'm also guessing that, rarely, you could find a silver weapon w/ alchemical silver on it - which would be the ultimate mud in the face along those lines.)
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