Used in the quest "The Siren's Tear". this quest is repeatable. any Fame can activate this quest.
Starting NPC: Wahid [I-8] Bastok Mines.
Reward: 150g + Title Tearjerker
WALKTHROUGH
1. Talk to Wahid.
2. Optional Step: Talk to Carmelo in the Steaming Sheep Restaurant in Bastok Port.
3. Go to Gustaberg North (the eastern side, off of Bastok Port.) Follow the road to the creek at J8/J9.
4. Remove any weapons or shields you have equipped, then examine the ??? along the creek. You will be asked if you want to pick something up, select Yes to obtain the Siren's Tear. If you have weapons or a shield equipped, it will slip out of your fingers when you try to examine the ???. The ??? moves randomly between 5 different places along the creek, so it can take a bit of running around to find it.
5. Return to Bastok and trade the Siren's Tear to Wahid to receive your gil reward.
(Original info from http://ffxi.somepage.com/questdb/26 )