Nothing indicates the item cannot be used as a component
There is no Sage Advice on this feature, so we are looking at official rules content from pulished books.
Creative Crescendo is a modification of the 3rd level Performance of Creation feature (partially quoted in the question), which also says this
The item glimmers softly, and a creature can faintly hear music when
touching it. The created item disappears after a number of hours equal to your proficiency bonus. For examples of items you can create, see the equipment chapter of the Player's Handbook. (...) The size of the item you can create with this feature increases by one size category when you reach
6th level (Large) and 14th level (Huge).
Creative Crescendo's full text says:
When you use your Performance of Creation feature, you can create more than one item at once. The number of items equals your Charisma modifier (minimum of two items). If you create an item that would exceed that number, you choose which of the previously created items disappears. Only one of these items can be of the maximum size you can create; the rest must be Small or Tiny. You are no longer limited by gp value when creating items with Performance of Creation.
This means that the item
- must be nonmagical
- glimmers softly, and a creature can faintly hear music when
- disappears after a number of hours equal to your proficiency bonus
- can at most be Huge (or Tiny or Small for subsequent items)
- has no limite to its gp value
So, at first look, there is no reason you would not be able to create a non-magical diamond worth 25,000 gp that glimmers softly.
The game rules simplify the value of objects, by assinging a fixed gp value to things such as jewels, clothing or silver ingots. In reality, the value of an item is always dependent on if there is someone willing to pay the price, but that is not how value of items is measured in game, even though it may influence what the PCs can sell an item for, independent of its inherent value. In-game, NPCs might be very well unwilling to pay much of anything for a suspiciously glimmering diamond that lets you hear faint music when you touch it, if they suspect that the object may be a transient magical fakery. Technically though, it still is a 25,000 gp diamond, even if nobody is willing to give even 100 gp to the characters to purchase it from them.
Creation. The first comparator is the creation spell. This spell cannot create usable material components, as it includes a statement that
Using any material created by this spell as another spell's material component causes that spell to fail.
This is a 5th level spell, while the feature, as a level 14 feature, would only be available to bards with access to 7th level spells. The feature lacks any such language that would block use as a spell component.
Minor Conjuration. The second comparator is the minor conjuration feature of the Conjuration wizard. This becomes available much earlier, and it is less clear if it could create components of value, as it makes no statement about the value of the created objects. The current consensus answer is that it seems to be allowed by the rules text, but should be expected to be ruled out by the DM, as it invalidates the cost of such components nearly entirely. The same reasoning could apply to Crescendo too, but Crescendo has explicit language about the unlimited gp value of the created components that this is lacking.
Wish. The wish spell, for a 9th level spell, allows this: "You create one object of up to 25,000 gp in value that isn't a magic item.". This is two spell levels higher, but in exchange the item has no suspicious glimmer or faint music, and it is permanent. The diamond here certainly could be used as a spell component, as this is again not excluded, and seems legitimate from a balance perspecive, considering that you can cast any non-9th level spell without need for its components, anyways.
Balance and playabiltiy
Few campaigns get far beyond to level 14. If you follow the rules, allowing this might allow the characters in a typcial campaign to get maybe a handful of uses out of it during the last level or two. It still is value, a typical PC would have about 57,000 gp in wealth by level 14, but since there are normally no magic items to buy, buying expensive components would seem like a natural use, meaning that getting them for free with this may affect balance less than if the PCs did not have access to such gems.
In the end, I think a DM should decide if they are comfortable with this, but there is noting in the rules that contradicts this being possible.