Gemstones with analgesic properties, so named for the yolk-like clay they form in. Because of their painkilling properties, they are considered valuable in some circles.

Before long, you’ve dug out a handful of yellow clay with the consistency of a cooked egg yolk. Crumbling this clay in your hands, you reveal several sparkling gemstones, pale yellow and wonderfully translucent. - The Hive (GS-ADV3)

Yolkstones can be found in subterranean rock that emanate a distinct smell from the surface;

You inspect the cavern walls and notice a streak of pale stone that smells like fresh grass, a clear sign of a yolkstone deposit. - The Hive (GS-ADV3)