Stellatite, also known as Star Stone was a precious mineral, supposedly the remains of a falling star. It was exceedingly valuable; it was known to have "otherworldly" properties. It was said that it could help combat hypnotism.
Malcolm the Healer gave Will Treaty a mineral of such to send to the imprisoned Alyss. Will shot the stone through Alyss' jail window so that she could use it while she was being questioned by Sir Keren with his blue gemstone.
It turns out the stone wasn't really Stellatite, just a black river pebble. Malcolm told them it would help so that Alyss would concentrate on something other than the blue gemstone.