Can a spellcaster use a focus not specified by his or her class?

Question

The question speaks for itself, but here are a few examples:

• My cleric's holy symbol is broken, but she still has an arcane orb.
• My bard wants to sing and speak instead of using instruments, and would like to use a simple amulet (rather than an instrument) for the material components.
• My wizard has a yew wand (a wand nonetheless, but druidic) and would like to use it to cast arcane spells.
• My warlock wants to use a quarterstaff instead of a standard staff as an arcane focus.

Are any of these cases possible? I think this might be a RAI issue, but if there are any hard references in the rules, they will be prioritized.

Answer 1

Spellcasters can only use the focus specified by their class, by reasoning that it is specified what kind of focus they can use. It doesn't say they can use a focus, it tells you exactly what kind of focus they can use.

If they could use any focus they liked then what would be the point of having specific foci for each class?

If that isn't convincing enough, then the descriptions of the foci themselves tell you what class can use them, as on page 151 of the PHB.

The only two that might be permissible is a sorcerer, wizard, or warlock using a wooden staff or yew wand - which are druidic foci - since a staff and wand are arcane foci and it's not stated that they have to be made of any particular material. However, an arcane focus is "designed to channel the power of arcane spells" so whether or not these two druidic foci will also work as arcane foci will depend on DM ruling.

Answer 2

No, but there's wiggle room

The druidic focus and holy symbol are well-defined in the PHB. They are written like exhaustive items in a list. But if you interpret the word "might" as listing items in a partially complete list, you could get away with anything (the DM willing).

Druidic Focus. A druidic focus might be a sprig of mistletoe or holly, a wand or scepter made of yew or another special wood, a staff drawn whole out of a living tree, or a totem object incorporating feathers, fur, bones, and teeth from sacred animals.

Holy Symbol. A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic

Meanwhile, an arcane focus is very open. It can be anything designed to channel the power of arcane spells:

Arcane Focus. An arcane focus is a special item -- an orb, a crystal, a rod, a specially constructed staff, a wand-like length of wood, or some similar item -- designed to channel the power of arcane spells

As for Bards, they need to always use a musical instrument. But the full list of instruments is not listed in the PHB, and so you have a wide latitude of defining what is a "musical instrument". Can you cast via music box? What about a wine glass (you can certainly make music with wine glasses)? What about a coin (since you can make music with a coin)?

I suggest to check with your DM on what they will consider instruments. A typical answer would be anything like drums, lutes, flutes, and the like. A creative flavor could be added like the examples above.

Musical Instrument. Several of the most common types of musical instruments are shown on the table as examples. If you have proficiency with a given musical instrument, you can add your proficiency bonus to any ability checks you make to play music with the instrument.

Answer 3

Addressing your examples,

• No, the Cleric couldn't use an arcane orb.
• No, bards must use an instrument as their focus. But it doesn't say they have to play it.
• Maybe. If it is also "designed to channel the power of arcane spells".
• You can't use a standard quarterstaff as an arcane focus staff. But you can use an arcane focus staff as a quarterstaff.
  A standard weapon-grade quarterstaff is just a 6' rod of wood. An arcane focus staff is a 6' rod of wood that is specially constructed to channel arcane spells. You can use either to hit someone, but only one to cast spells.

And, there's always the option of the component pouch, which can be used by any casting class.