Can the School of Conjuration wizard's Minor Conjuration feature (PHB, p. 116) be used to summon rare, expensive, and/or consumable spell components?
It seems to fit within the scope of the feature in a rules-as-written manner, and I am inclined to allow it - but not having fully experienced the dynamics of higher level play where it might come more strongly into play, I have some uncertainty.
Yes. There is nothing in the description that says that you can't use the created object as a material component. The only qualifiers are related to size and mass, and that the created object be nonmagical. Since material components are not typically magical, you can create them with the class feature.
I'd be very wary of allowing this kind of thing. Material components on spells exist for two reasons: reduced availability, and opportunity cost. In order to cast a spell with an expensive component, you need to have purchased that component at some point before, which means that you need to have thought about the number of times you wanted to cast the spell beforehand. Also, you have the opportunity cost of having to tie up some of your wealth in spell components that you may or may not ever use. Allowing Minor Conjuration to make expensive material components totally bypasses both considerations, and makes them basically pointless for a Conjuration Wizard. It feels like if the class feature was intended to bypass a system like that, they would have said so.
That said, this is a game of rulings, not rules. Maybe you like the idea of a world where conjuration wizards can make material components out of thin air. Maybe there's a shop in every city where a conjuration wizard sells half price spell components that you have to use right then. Maybe a cabal of conjuration wizards and their simulacrums secretly runs the world with expensive magic that the rest of the world has to pay for. If that's the world you want to play in, then by all means, allow conjuration wizards to bypass expensive spell components.
I agree with DuckTapeAI's answer that nothing in the Minor Conjuration feature itself prevents one from using it to summon an object later used as a spell component. However, sometimes this can be disallowed by the nature of the component itself.
For example, the since-deleted question level 5 double summoner (which was briefly linked as a duplicate to this question) asked about using Minor Conjuration to produce the material component for a Glyph of Warding spell. In this specific case, Minor Conjuration would not be allowed, because "powdered diamond" is not an object.
The Minor Conjuration feature allows you to summon a single object:
you can use your action to conjure up an inanimate object in your hand or on the ground in an unoccupied space that you can see within 10 feet of you. This object can be no larger than 3 feet on a side and weigh no more than 10 pounds, and its form must be that of a nonmagical object that you have seen.
The DMG (246) tells us that an object is:
a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
Glyph of Warding has as its component:
incense and powdered diamond worth at least 200 gp, which the spell consumes
Ignoring the incense, the fact that the diamond must be powdered disqualifies it from being summoned by Minor Conjuration. Since the powdered diamond is composed of uncountable individual grains of diamond, it violates the "discrete" part of the definition - at best, each individual grain is one object, and only a single grain could be summoned, but a single grain is not worth 200gp.
One could certainly use Minor Conjuration to summon an intact diamond, for example the 300gp one needed for Revivify. However, one could not then grind such a diamond to powder. The DMG tells us, in the section on objects (246, 247), that:
An object's hit points measure how much damage it can take before losing its structural integrity.
Grinding a diamond to powder destroys the structural integrity of the gem, which by definition means you are doing it damage. And one of the conditions of the Minor Conjuration feature is that:
The object disappears...if it takes or deals any damage.
Thus Minor Conjuration is fine in the case of spells whose component is actually an object, but for those which need powdered gemstones, such as glyph of warding, it will not work.
Logically and by RAW, there is no problem with this, as the effect clearly states you can replicate any nonmagical item in certain sizes. However, if you need a reason to ban the level of effect that uses several thousand gold pieces in components, you could change some of the spells material components so that they need to be enchanted, or larger than a conjuration wizard can produce. Otherwise, it seems rather harmless to the game to let him create a tarot deck for augury, or a tiny diamond for chromatic orb, especially if it only lasts an hour.
