Morgause's crystal gave the sorceress Morgause the ability to view things that were happening in the present.
Morgause was first seen looking into her enchanted crystal while she was observing Arthur in her ruined castle. She leant over the crystal whispering Arthur 's name and a spell ("Hider eft funde. On þisse ne middangeard"). She then said: "A great destiny awaits you. The battle for your soul is about to begin" (The Sins of the Father ). In the end of the episode she looked into her crystal again, chanting the spell "Ætíe mé þá þé ic séce", probably to see how the fight between Arthur and Uther had ended, hoping that the king was dead, but she only saw her two enemies sitting together, joking and having fun. She then looked up with an expression of shock and anger in her eyes (The Sins of the Father).
Morgause is then seen watching Cenred's armies preparing to invade Camelot from her cave with satisfaction (The Tears of Uther Pendragon).
It could be that this crystal was hewn from the Crystal Cave, as it bears resemblence to the Crystal of Neathid, which Taliesin said was hewn from the cave (The Crystal Cave).