I did manage to beat the Incubus on B6 and got the Incubus Wings. Because the Incubus is woven to the story I don't think I will ever see another one and therefore get no other wings. I also can't sell it to the material shop to get more. So, it's only a single use and I want to make the best out of it. Is a magic stone the best I can do with it?
The best use for them is to make new stones at the Vigger's Shop (for characters' spell-use upgrades) or even just use them, rather than bring them to the Materials Shop.
*** Important reason: Incubus Wings are one of the three unique materials in the game (the other two being Golem's Flesh and Sword Fragment). If you play long enough, then you'll almost certainly bring every type of item to the Materials Shop, and bringing Eric those 3 unique items will finish Lydia's quest, and he'll close down the shop -- which means that any further spell stones will require painfully slow level grinding to obtain the requisite components. When your PCs have reached max level, that level grind becomes tedious because they're not getting any more powerful, they're just hunting for material over and over. Simply buying the components at the shop and using them to make stones at the Vigger's Shop in town is only a short trudge, and at high levels you'll have plenty of spare cash to make stones and maximize your spell casting capabilities.