According to this post by Jeremy Crawford, a Night Hag's Heartstone is meant to work for hags but it is a DM's choice whether it works for PCs.
The Monster Manual (p.177) describes the Heartstone's abilities as:
Heartstone: This lustrous black gem allows a night hag to become ethereal while it is in her possession. The touch of a heartstone also cures any disease. Crafting a heartstone takes 30 days.
Does that rule-it-out or rule-it-in as being Adventurer's League legal for PCs to own, use and or trade?
We are looking for RAW/RAI guidance or precedent. Our assumption is that etherealness is only usable by the night hag – but Cure Disease would be usable by PCs.
This is particularly relevant currently as the AL adventure (spoiler)…
Tomb of Annihilation has at least one hag who carries a Heartstone thus making it possible for PCs to obtain one.
Best Answer
I would say no.
The rule text specifically states "...allows a night hag to become ethereal" and the night hag has an action called "Etherealness", which requires a heartstone.
The stone does not provide this ability, it only enables the night hag to use her ability.
On a more balance-related note, it would be an amazingly good and game-breaking item. It's unlimited castings per day of Etherealness (a level 7 spell).
As for the cure disease effect... Looks like it would work, RAW. Not terribly game-breaking either (though most diseases would obviously become a non-issue).