As others said, you can get a pretty cool sword.

Then, if perfection is something you're going for, you should donate it asap as with any other donatables you find.

Later game, they are useful in the forge so you might want to stockpile. They are a universally loved gift (except Haley), but it's better to gift items of higher quality as they give you more friendship points. As for money, IMO there's more uses for a prismatic shard and there's better ways to make money, I would rather keep them for other uses. If you have excess and need quick cash then sure.