The piece of the Stone Disk belonging to the Dark People. Unlike most of April's encounters with the magical races, they gave her the stone, along with the Star Map, without much rigmarole. Given what she's already been through, she finds this quite a relief.
The stone itself is shaped like a flame, presumably the symbol of the Dark People, as they are 'keepers of the Dark Flame'.
Given to April by the Dark People's emissary on the Shadow Ship.