Path of Exile has been absorbing a little bit of my time, not as much as I should probably be dedicating to it I'm afraid. It's hard for me to get super 'into' games now. I often end up thinking about tabletop roleplaying games in conjunction with them, and this is why I thought of skill gems.
Stats and Gems
If you wanted to be a purist, you could boil down D&D's stats into just 3; Strength, Dexterity, and Intelligence. Similar to what Into the Odd has done. Or if you wanted to be a really cool guy, you could keep all of D&D's stats and make up new kinds of gems for the extra stats, which might be even color.
Strength gems are red, Dexterity gems are green, and Intelligence gems are blue. If you add in the other stats, make Constitution gems orange, Wisdom purple, and Charisma yellow. You can change the colors around if you prefer, doesn't really matter.
You can only use a number of gems equal to the relevant stat's modifier +1. So if you have +2 Strength modifier, +0 Dexterity modifier, and -1 Intelligence modifier; you can equip 3 Strength Gems, 1 Dexterity gem, and no Intelligence gems. You also have to socket it into an item to make use of it.
Support gems also count as the above and towards the limit of gems you can use, but support gems can support any type of gem as long as its equipped in the same item your other gems are.
Using any gem costs 1 mana point. Your maximum mana is equal to your level + Wisdom modifier. Every support gem attached to a normal gem increases its mana cost by +1 per use.
 Heavy Strike- Add 1d6 to the damage of the attack. If any die rolls maximum you can knock the enemy back by 5ft or stun for one round.
 Cleave- Attack up to 3 targets in a cone in front of you.
 Shield Charge- Hold out your shield and move at double speed for one round, knocking enemies aside. You can also aim at a target and slam them, dealing 1d8 damage to that enemy.
 Vitality- All allies within 15 ft. of you restore 1 point of health each round, as long as they are attacking something.
 Static Strike- Every time you hit an enemy, you gain one static counter. When you choose, you release all counters on you and deal 1 lightning damage to all enemies next to you per static counter.
 Molten Shell- Increases your AC by 2 for up to 3 rounds. If anyone hits you while the shield is up, there is a 50% chance that it will burst on them, dealing 1d6 fire damage.
 Reckoning- If an enemy attacks you and the damage is blocked by your shield's AC, you perform a blast against them that deals 1d4 damage.
 Vigilant Strike- Cast immediately after hitting an enemy. You take -1 damage from all attacks and spells for 3 rounds.
 Infernal Blow- Add +1d4 fire damage to the attack, if you kill an enemy with your total attack damage this round, they explode and deal 1d4 damage to all adjacent enemies.
 Leap Slam- Jump up to 30ft in any direction. You can make a free attack against an enemy you are landing upon.
 Purity of Fire- Grants +2 to all breath weapon saves and +2 fire resistance to yourself and all allies within your eyesight.
 Decoy Totem- Creates a totem that enemies must make a save or must attack before other party members. The totem has your AC but can only 2d6 HP.
Dexterity Gems – 1d12
 Animate Weapon- Allows any weapon you currently own to fight by your side. The weapon makes normal, unmodified attack rolls. After getting hit by an attack or making 3 of its own attacks, it falls to the ground now mundane.
 Poacher's Mark- Curses a group of enemies under the same banner or of the same type you are fighting to have -2 AC. If you or an ally kills one of them, they restore +1 HP and the effect ends.
 Spectral Throw- Ghostly projection of your weapon flies forward, making an attack roll against each and will deal normal damage on each if it hits. Enemies can also spend their turn avoiding it, but it rebounds and can hit enemies a second time.
 Barrage- Stand still and fire off arrows at an incredible rate, letting you attack 3 times with a ranged weapon in one round. You cannot turn more then 90 degrees while firing.
 Reave- After hitting an enemy in melee, you can attack another enemy as long as they are adjacent to the first enemy. Every time you chain attacks like this, it costs +1 mana point growing as each enemy is hit. So 1 mana for the first chain, 2 more for the second, 3 more for the next enemy, etc.
 Puncture- Enemy begins to bleed dealing 1d4 damage a round until they make a save. No effect on enemies without blood or equivalent.
 Temporal Chains- All enemies within a radius of 25ft you dictate move half as far, have half the number of attacks, and have half initiative scores. If they have curses or debuffs on them, they last one additional turn.
 Viper Strike- On your next attack enemy must save vs poison or take 1d8 poison damage.
 Grace- Aura that grants +1 AC to you and all your friends within shouting distance.
 Mirror Arrow- Creates a duplicate of yourself wherever you shoot your bow, and both of you may make attacks for one round.
 Ethereal Knives- Fan of knives that deal 1d4 damage to all enemies in a 15ft cone from you.
 Lightning Arrow- Arrows that deals additional 1d4 lightning damage. If you are outside, 50% chance to call actual lightning bolt on hit that deals 1d10 damage instead. If in a storm, 100% chance.
 Flameblast- Charge up flaming energy on a single 10ft spot. Each round you charge it costs 1 more mana and deals Xd6, where X is equal to the rounds charged. The spot cannot be changed once you begin casting this spell.
 Freezing Pulse- Deals 1d6 damage to all enemies within a straight line of you, and on a roll of 6 their feet are rooted to the ground with ice, making them unable to move for a round.
 Herald of Lightning- Add +1 lightning damage to all attacks and spells cast to all allies within 10ft. If an enemy is killed by dealing at least 15 damage to them in a single attack, they explode outwards with static bolts that deal 1d4 damage to all enemies within 10 feet of the first enemy.
 Righteous Fire- You and all enemies within melee range take 1d4 fire damage per turn. While this effect goes on, you get to add 1d4 bonus fire damage to all spells you cast.
 Enfeeble- Curses all enemies that you can see at the moment you cast the spell. The first damage dice they roll for any attack, skill, or spell is treated as a 1 before modifiers and bonus die are added.
 Assassin's Mark- Curse a single enemy to be marked for death. The next time you or an ally attack them, add your sneak attack bonus to the attack and damage rolls.
 Essence Drain- Fires a projectile that drains the victim of 1d4 levels or HD, recovering them at 1 per round after being struck. They can make a save to avoid.
 Lightning Tendrils- Make an attack roll with advantage against all enemies in a cone in front of you. Each enemy struck takes 1d8 lightning damage.
 Power Siphon- Fires a bolt that deals 1d4 damage to an enemy. If you kill an enemy with this, you deal +1 damage with all spells for the rest of this combat.
 Summon Skeleton- Creates a weak skeleton minion that lasts for 3 rounds. Any Turn Undead, Stun, or Curse effect ability used on it destroys it instantly.
 Elemental Weakness- Curses a group of foes fighting under the same banner or of the same type to take 3 more damage against an element you specify when you cast this spell. Lasts 4 rounds.
 Discipline- Your allies and yourself all ignore the first point of damage you take each round.