Brisingamen is the 150 Karat brilliant diamond which is part of the necklace adorning the statue of the goddess Freyja in her temple on Hindarsfjall. According to the slightly exaggerated opinion of Jarl Crach an Craite, the diamond is worth almost as the all shipyards in Cidaris, including employees and annual production.
Spoiler warning: Significant plot details follow.
Yennefer needed to use Brisingamen to construct a megascope in order to communicate with some people necessary to find Vilgefortz' hideout.
Significant plot details end here.