Keep in mind that break siege happens before storm castles. So, that’s two battles in one turn. That’s where gem baiting could happen.
As for triggering gem use, that’s harder these dags. You need a strong enough thug. But, if you load an air mage with 4 trinkets, that should do it in many cases.
As I said, Illwinter changed it from the days of Dominions 3 and 4, when it was super easy to bait gems just by attacking with scouts.
Most games banned it back then. Many games still ban gem baiting, even today (even though it’s substantial harder to do).