My first attempt at turning a Mallee cap. These things are hard as rocks! Picked it up today when visiting Bad Dog Burl Source and turned it when I got home. Finished with Bush Oil. Around 6" (15 cm) across.
Paul, I used a woodworm screw in my chuck to turn a flat with a shallow recess on the bottom, then expanded the chuck jaws into the recess to turn the top. Kept the live center up against the piece for most of the turning of the top.