As I understand it, silvered (PHB p. 148) and/or adamantine (XGtE p. 78) weapons or ammunition do not automatically count as magical. There is no text describing them as magical, and they are not found on lists of magic items (unlike, for example, Mithral Armor or Adamantine Armor which appear on a list of magic items in the DMG).
I mention this because the spell Swift Quiver has some text that concerning nonmagical ammunition (PHB, p. 279-280, bold added):
Components: V, S, M (a quiver containing at least one piece of ammunition)
You transmute your quiver so it produces an endless supply of nonmagical ammunition...
Each time you make such a ranged attack, your quiver magically replaces the piece of ammunition you used with a similar piece of nonmagical ammunition.
So the ammunition that is produced by this spell is nonmagical, but otherwise is "similar" to whatever piece of ammunition was just used. And the quiver starts with at least one piece of real ammunition in it (that you used as part of a material component to cast the spell, and that is not consumed in the casting). So that got me thinking:
If someone was to have a quiver that contained one silvered arrow and one adamantine coated arrow (neither of which was magical), would they then be able to use Swift Quiver to attack with silvered (or adamantine) arrows until the end of the spell's duration?