Ultima 6 - who knows where to find the vortex cube

The caretaker in the Hall of Knowledge mentions that the cube was stolen from the gargoyles by two humans thieves and that they were heard mentioning Stonegate as they fled.

This is the relevant line:

cham,vort,libr,huma: "To have three rooms here:" "To keep artifacts from the Underworld in the Human Chamber, to the east." "To keep valuable books in the Librum Chamber, to the west." "And to keep the Cube and the Lens in the Vortex Chamber, to the south."

cube: "To regret that that item is lost, stolen by human thieves years ago." "To have heard the humans mention the name Stonegate as they fled." "To have heard them say they would sail through Lost Hope Bay." "To know nothing more about its whereabouts."

So, once again, the expected plot would probably have you look in the Hall for the Cube, find out it is no longer stored there, ask for info and get the clue to Stonegate.