Talk to Wahid; he's a collector of precious gems and is looking for a Siren's Tear. He says they can be found near the rivers of the Gustaberg Mountains but will slip through your fingers unless they are handled in a special way. Also, there is a bard in a tavern somewhere who has advice on how to get the jewel.
(Optional) Talk to Otto in the Steaming Sheep; he will point to the bard mentioned above, Carmelo.
(Optional) Talk to Carmelo; he sings a cryptic song explaining how to get the tear and he explains its meaning.
Go to North Gustaberg (east section) and locate the ??? near the river at J-9. (You can avoid aggro by going via Port Bastok.) The ??? will move around the shore line, so it's not always in the same spot. A good strategy is to check one side of the river, then the other until you find the ???.
Unequip your weapon and shield and examine the ??? to pick up the Siren's Tear.