What rarity is a Night Hag's heartstone?

[Koningkrush]

I'm running a campaign where the party managed to kill a night hag with true sight (knowing it was stalking them), then an ambush silence spell + group-grappling + restraining, in addition, to quickly stealing its heartstone before it was able to go Ethereal, killing it as a result.

They will likely attempt to sell the heartstone, and they are in a location that is loaded with disease (will probably end up doubling the value of the heartstone). The place they are selling it is absurdly abundant in wealth and trading with people from all over the world circulating through the markets.

What I'm uncertain about is what to price the heartstone for as far as magic item rarity goes (Uncommon, Rare, Legendary, etc.)

(Edit: Well, I'm apparently blind as a bat.
"A night hag carries two very rare magic items that she must craft for herself.")

[Asmotherion]

Well, from what you're saying, my thoughts are the following. However, due to thinking a lot about RP implications of things, my thoughts may or may not contain some spoilers. If you don't want to be affected in your RP by my thoughts, don't oppen the spoiler.

-Did you or any of the other characters know the value of such an item?
-Did your characters know anything else significant about it?
-Perhaps the fact that the actual rarity is not mentioned to your group is a hint from your DM about something?
-Remember that Hags are users of Dark Magic, and that their magic usually is baleful. Perhaps the sick people is a side effect of the Heartstone being used, and everytime it is used to cure someone, someone else gets sick (one example).

Just saying...

[ad_hoc]

For rarity you just need to decide how many there are. If there aren't very many of them then they're pretty darn rare.

For value you've got to decide how valuable they are to people. If what they can is both needed and can't be done elsewhere then they're probably pretty valuable.

[SirGraystone]

As a DM playing the buyer, I would start by asking the player selling it how much they want, then offer 20% of that, then let the negotiation begin.

[BobZan]

If they want to trade it for other Magic Items, there are new rules on XGtE. They are cool.