G'axir would like you to bring him a Sphene Gem for what purpose he will not say. You acquire this quest by speaking to G'axir the Seer in the Blade and Stars tavern in Baldur's Gate. After providing some foreshadowing of your heritage and terrible fate he tells you that he wants you to bring him a Sphene Gem and suggests getting one from the Greater Basilisk in Baldur's Gate. There are many Sphene Gems in the game but, while any of them will do, taking the one from the Basilisk is convenient. This Basilisk is also the target of the Basilisk on the Loose quest. It is located in the northern warehouse in Baldur's Gate Docks.