"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
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 the all shipyards in Cidaris, including the employees and annual production for a year.
Yennefer needed to use Brisingamen to construct a megascope in order to communicate with some people necessary to find Vilgefortz's hideout. High priestess Sigrdrifa refused at the beginning but than she allowed it for the good of Ciri.