Well, went for the unarmed disarm and was successful! So my character now has a negative level (from holding the Chaos Diamond) and the half-demon sorceror cast Flesh to Stone on him instead (DC 26 Fort save, which he made with a +23 modifier). So my character's next trick is to try and disarm or sunder the spell component pouch to at least take the sorceror out of the fight (or is that the fight out of the sorceror?), but I think I'll start a new thread for that one. These tactics are new to most of us, as we typically just fight a war of attrition, so I appreciate the advice.
Edit: Good point Domino about the brittle nature of diamonds. I'd forgotten about that. And no doubt we'll be making use of that nature (assuming we survive) as no one in the party will tolerate such an item and the wizard can use it for spell components (he has Stoneskin memorised most days).