Jane, the inside diameter (ID) must be smaller than the girdle of
the stone and the outside diameter (OD) must be larger. It's that
simple. The bigger the stone, the thicker the wall should be to
allow for the metal that will be cut away forming the seat. I would
not set a large or expensive stone in ready made tubing unless the
tubing was made with that purpose in mind and had really heavy
walls. You can form up your own tubing from fine silver and so
control the wall thickness.