The Upgrade Gem is a collectible unit that enhances the firepower of the player's ships and energy collection. Each Upgrade Gem collected adds 20% to the extra power, up to a maximum of 100%, after which collecting further gems does not improve the player's capabilities.

At 100%, weapon range is increased by 50%, weapon delay between shots is reduced to 50% (firing twice as often), and energy production is increased by 50%. It seems to go linearly through the steps, so all of the bonuses are half of what the listed gem % is. So if you've picked up two gems and it says 40%, all of the bonuses would be 20%. It is possible this may vary slightly between weapons, or that there are other effects as well.