For those of you wondering how Gem Boosting works, I'll try to explain it the best I can. When hovering over the Gem Boost tab it will display all of your current gems listed in your inventory. Each gem gives the current ore a boost to health and defense depending on grade of the gem selected. Once you click the gem, it will be consumed (maybe dev will implement multiple selection option and a confirmation option about gems selected). Once the gem is consumed, you will notice that the HP/Def of the ore has increased. The given XP% and overall drop rate % will also increase. Thus, making a large increase to overall gains. Hope this helps!
You can change your main character using either Seals of Leadership, or Medallions of Compromise.