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?
Best Answer
Yes.
In order to cast the spell, you must provide the following material component (from the spell description):
If you have a quiver containing a silver arrow and an adamantine arrow, you fulfill the material component requirements of the spell. In other words, the quiver of at least one arrow (not simply an arrow by itself) is the material component.
Then, here's how the spell's effect works (from the description, emphasis mine):
Once the quiver is transmuted, each attack you make using ammunition from the quiver causes the quiver to automatically replace that ammunition with a similar sort. Since you make multiple attacks during the spell's duration, each attack with an original arrow produces a replacement arrow, which then gets used and replaced, which then gets used and replaced, and so on.
As you correctly noted, silver and adamantine ammunition are not inherently magical, so each replacement arrow will be of similar make. But, do note the following (from the description):
So once you've shot one of the original pieces of ammunition, no matter how many times you can continue to replace and shoot the replacement again for the duration of the spell, once the spell is over the original (which you've shot away) and any of its replacements (which disintegrate) will be gone. Thus if you start with only a silver arrow and an adamantine arrow and you fire both silver and adamantine arrows during the spell's duration, you'll be left with an empty quiver (unless you can recover the original arrows subject to the normal ammo recovery rules).