Can the Minor Conjuration ability gained by Wizards of the School of Conjuration (PHB p. 116) be used to summon rare, expensive, consumable spell components? It seems to fit within the scope of it in a rules as written manner, and I am inclined to allow such, but not having fully experienced the dynamics of higher level play where it might come more strongly into play, I have some uncertainty. What do you think?
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.
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.
If I were dm'ing I would allow it make spell components that normally have a gold cost (diamond,jade,pearl,ect) but enforce a few exceptions.
First it has to be made of the same material through out. You can have a jade gemstone but not a crown with gems for instance.
Second you couldnt conjour a powder or grinded up material.
Lastly and can only be used with spells that will not consume the item. So you could make a diamond for chromatic orb but not a diamond for use in continual flame.