Understanding the verbiage used in the descriptions of spells is important. Removing gems/rows/columns does not destroy them, so no one gains mana and no opponents are damaged by removed skulls. Destroying gems/rows/columns blows up only the gems as described in the text, which means your team gains any mana coming from the destruction and skulls do damage. Exploding gems takes out the gem you select, plus any surrounding gems, resulting in incoming mana and outgoing damage.