Well, what it says on the title. The item Dust of Disappearence, sold in the academy in shadowscape by Sayanran Lotkin (possibly other places), which casts improved Invisibility is not stackable and it would be great if it was.

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Playing as:
Aleana Xiloscient: Wherever the winds take her.
Jealesyl Truesong: A voice in the dark
DM Prometheus: Bringing you fire

+1... yes please.

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This isn't mechanically possible - the item type isn't one that can be stacked.

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Davion Telemos - Monk of the Four Winds
Korthan Isharnos - Dragon Shaman of Thunder Spirit Zamasham

how about the black balls (cast darkness) and the gas bombs. Can those be stacked?

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MuseReader: Aiseth Nosdivan- Master Enchantress

No for both, far as I remember.
This is possible (for all three, plus anything else), but it requires changing the base item, then it's just a matter of making sure that item referenced by the shops that sell them is drawing from the updated version. Existing versions would not be updated, but new ones purchased would be stackable. So if the Team decides its not unhinging anything to make more items (such as these) stackable, it's a relatively quick and minor thing to change; really the only thing is if it would be approved or not. Does it effect balance or whatnot in some way, allowing people to carry a bazillion of these things, instead of being limited to a more finite amount due to their size and thus inventory use, etc...