Planar Binding Fiend Core
Recovered from Azzrukhan the Banished Blade.
A smoky brown quartz, split into 3 by Rai'Voa. When removed, it caused the Flayhorn's body to pseudo-combust.
These pieces of smoky quartz are, together, a powerful artifact, and an ancient one at that. Someone, likely very long ago, imbued this stone with Abjuration magic. Particularly, a version of the Planar Binding spell.
Riga feels that the very crystal is bound to the Material, not just creatures stabbed/holding it. The shards, together, feel like a part of a larger whole.
To Riga it feels kind of like when you are looking at half of an item, like one of two magic Boots
Additional Research
Smoky Quartz has been associated with Kelemvor on some occasions due to its foggy appearance, but it's not as official as Amber being Kossuth's gem. Riga had also heard of it being used in some magic items or rituals tied to 'purification of bad energies', whatever that meant.