All forms of ceramics can
All forms of ceramics can work – unglazed is best. If glazed, sandblast to roughen the surface and kilnwash. Deeper shapes may require drilling air-escape holes.
Stainless steel (also kilnwashed) is good for draping over – not into unless very shallow.
Avoid rocks which can have water trapped inside which, when heated, will expand and cause the rocks to explode.
Helios Kiln Glass Studio