Does the staff version of an arcane or druidic focus also work as a quarterstaff?
At least at character creation can you say that for example your druids quarterstaff and druid focus staff is one and the same?
Logically you'd think so, after all a long heavy staff is a long heavy staff and there are plenty of examples of magic staffs doubling as quarterstaff from Gandalf to Harry Dresden.
But is there anything in the rules that say it is or isn't allowed?
I asked Mike Mearls:
Can the arcane/druidic focus staff double as a quarterstaff?
And received the following response:
yes
Any Staff, magical or otherwise, can be used as a simple melee weapon in an identical manner as an ordinary Quarterstaff.
DMG page 140 states this regarding magic staves:
Unless a staff's description says otherwise, a staff can be used as a quarterstaff.
There are however other logical reasons for reaching this conclusion if the DMG excerpt is unsatisfactory:
Probably.
The entries for Staff and Quarterstaff are different in the equipment list. Under the Arcane Focus section you get:
a specially constructed staff (p. 48).
this is distinctly different (and quite a bit more expensive) than a typical quarter staff (the arcane focus version runs 5gp vs 2sp for a quarterstaff). So no, a quarterstaff cannot double for an arcane foci staff, or the other way round (at least without further clarification).
In the Druidic Focus section you get
a staff drawn whole out of a living tree (p 151 PHB)
The druid is much closer to nature and uses a druidic focus rather than an arcane one. We see that the staff for the druidic focus has a far more...utilitarian description. There is nothing here that makes it not a quarterstaff (though it may be rougher than a typical staff). My guess is most Druids would have no problem wielding their staff as a quarterstaff. I mean who is going to argue with you, you have a quarterstaff.
Last thing, there is an argument to be made for allowing implement staves to be treated as quarterstaves under the improved weapon rules on p 46 of BD&D
In many cases, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the DM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.
This means that if your staff focus resembles a quarterstaff (for some value of resemble I guess), then it could be treated as one and your proficiency bonus would apply to attacks with it (and it'd get a d6 for the damage die).
On your own accord
D&D 5th edition is trying to let players and DM handle much of these kind of "rule holes" as they feel right. Personally I would let an Arcane Focus shaped like a staff work like a weapon, but not the other way around (not every staff you find lets you focus magic). Also, regarding improvised weapons on the Basic Rules, it says that an object that resembles a weapon could be treated as such on proficiency bonus and/or damage dice matters.
The funny thing about this "ambiguity" is that you can add flavour to your games ruling things your way as you and your group feels ok. You may say there is no problem using the Arcane Focus as a QuarterStaff (as Gandalf and Harry would approve) or your setting could be one where "magical" staffs are very fragile and elaborate objects that shouldn't be used to hit anything.
On the same account, you can also play a middle way (as I said, like you feel is more interesting) letting these kind of objects be used as weapons and rolling the next time they are used as spell focus for the chance of it to have broken, maybe letting the character try to "shove it" (a la Ron Weasley) for it to finally work. I think that would be hilarious.
When improvising a weapon your DM can rule that it uses the stat block of another, similar, weapon:
Often, an improvised weapon is similar to an actual weapon and can be treated as such.
A wooden staff is akin to, if not identical to, a quarter staff. As you note: "a long heavy staff is a long heavy staff".
As was said, 5e allows for a lot of ambiguity and freelance such as running out of arrows and strangling someone with your longbow (I had a DM once allow me to undo one end of the string, attach a hook and use it as a fishing pole. Got a +1 Inspiration to boot.) The effectiveness and damage would be calculated by the most comparable item/action. If someone smacks you upside the head with a staff of arcane focus, you're going to agree it feels just like a quarterstaff (but a quarterstaff is most certainly not an arcane focus). As a DM you could also consider that while it swings like a quarterstaff, hits like a quarterstaff and hurts like a quarterstaff ... an arcane focus staff might be created to be more ornamental, not as resilient, and the price of using it for melee might mean a good critical miss would break it in to firewood.
