By "casting foci", I am referring to arcane focus, druidic focus, and holy symbol, because they're all just different fluff for the same thing, just keynamed for class restrictions.
When I say magic item, I mean magic in the sense that it has supernatural properties, not "magic item" as a rarity based loot-drop type item as exemplified by the DMG treasure lists. (That said, the second situation below may be affected by that definition) When I say mundane, I am referring to an absence of magic, not a state of boringness.
These are all listed in adventuring gear, and each type of casting focus gives examples of objects which can be used. The adventuring gear consists of mostly mundane objects, and the examples given sound like mundane objects as well. However, the list also includes magic items, like healing potions... and the foci are indeed being used to generate magical effects in the game without the use of non-monetary spell components. The answer to this question is relevant in these situations:
A character finds some need to improvise a casting focus, so they pick up an object which satisfies the description of an example. (Let's say, a wooden dowel rod for an arcane caster who lost his wand) Does it work? If so, why can't an arcane caster use any phallic object, (most weapons, his finger, or even his actual phallus) as a stand-in for the word "rod"?
A character, through whatever circumstances, loses their focus, so they decide to make a new one with downtime. Should they use Craft or Create a Magic Item? If they must use create a magic item, what is a druid doing that is so special when he cuts this sprig of mistletoe over any other sprig of mistletoe?
This is based on me thinking about this answer I gave a while back, and being dissatisfied with it. (And a friend made some dick jokes about the word "rod" as an example for arcane foci.) I cannot find anything in the core texts which states clearly one way or the other. (Though I've been known to be a poor reader) So, I would like to know,
is there any official source which clearly states whether or not casting foci are magic items?