This stone radiates strong magical energy.
The Seeing stone is found in the Salamandra hideout in the restricted part of the Trade Quarter. It is used to activate a telecommunicator, Hartmann's mirror, which is found, along with a power stone, at the hideout when Geralt is investigating on behalf of Leuvaarden. According to Triss Merigold, seeing stones retain faint traces of previous communications which can be detected and analysed with the right knowledge and skills. Naturally, the sorceress possesses exactly such knowledge and skills.
There are two seeing stones to be found in the game. The one described above and a second stone, in the cave with Alvaro's portal in the Salamandra base. This second gem is in a small area opposite the portal and guarded by a Salamandra mage who, like his compatriots, loves nothing better than to summon ifrits.