This gem is the hardened tears of Sereness.  After witnessing the death of the pirate captain, Giler, with her own eyes, she is said to have shed tears, lost in the flow of time.  This gem contains her sorrow, love, and resentment.  She never even came close to the shipwreck since then in order not to stir memories of Giler.  Many of her tears are said to be still resting near the shipwreck.