The infinity gems are six special stones which each have a power. In Marvel Super Heroes, your character can use the gems they have to give them such things as a speed increase or more attacks. The standard 10 characters each have a preferred infinity gem which grants them a unique ability (these can be seen on each character's individual page).
To use a gem, simply do a quarter circle back motion followed by two punch buttons together. The effects of a gem do not last that long, but can easily turn the tide of battle. If you have more than one gem in your possession, you can cycle through to the one you want to use by pressing the start button. You can make an opponent lose their currently select gem by using certain attacks -for example, Magneto's magnetic tempest super move. One last note is that if you use a gem and the opponent is right next to you, they will take slight damage and get knocked back, unless they block.
MIND GEM
This gem is extremely useful as it will automatically fill up your super combo gauge. It will give you almost 2 bars by the time the effects of the gem stop.
POWER GEM
Does exactly what it says: gives you more power. With this gem activated, go on the offensive as all of your moves will do more damage.
REALITY GEM
This gives your character access to some projectiles. LP and LK will throw fireballs, MP and MK creates ice spikes and HP and HK will throw electric spheres.
SOUL GEM
This gem will make your life bar refill, you will get around 25% of your life bar back.
SPACE GEM
This makes your character much more resistant to attacks, so if you get hit you take less damage.
TIME GEM
This will speed your character up, meaning that all of their moves will come out faster.