Some gem basics

To begin with, there are 3 types of gems: fire, water and air gems. There are also small and big(empowered) gems, the main difference between these two is that the big ones drop only from Empowered Gem Box (you can recieve it from Legendary Tome: Empowered Gem Box Edition, weekly contests or by crafting them from Lunar Soul in Shadow Market). Big gems can only have radiant or stellar quality.

Each gem should be put into its own spot. Small slots are for the normal gems and empowered slots can only have empowered gems in them. You also can't have 2 same empowered gems(their names should be different) because each empowered gem has its own power. There are also class empowered gems which can be obtained by using a Class Gem Key in uber-8 and uber-9 world on the chest that spawns after completing a dungeon in Shores of the Everdark . They will have stats that only suit the class you are using right now (no PD for magic classes) but they will also have special "power" that will boost you character a lot.

• Boomeranger: Big Bomb becomes a Bawk-Bomb. It still sticks to surfaces and explodes in the same area, but also spawns several chickens that attack enemies for a brief duration.
• Candy Barbarian: Sugar Crush now leaps straight up and crashes down, vacuuming enemies toward you and applying a brief snare.
• Dracolyte: When a Burnt Offering detonates, it has a chance to spawn a mini Dracolyte minion that attacks enemies for a brief duration.
• Dino Tamer: 1)Changes the Hunting Dart from one dart to three. 2)Increases the duration of the ultimate ability, 'Summon Dino Mount.' Increases the attack speed of the ultimate ability, 'Acid Spit Projectile.'
• Fae Trickster: Blink decoys are no longer immobile; they close on enemies and explode on contact, dealing damage and stunning enemies.
• Gunslinger: Run and Gun no longer increases your attack speed, but all shots fired are max charged charged shots.
• Ice Sage: Ice Crash now fires a fan of icicle projectiles, which apply a stacking snare. When an icicle hits an enemy with a full stack of snare effects, it spawns a nova of ice shards from the enemy.
• Knight: Charge no longer propels you forward. Instead, a Spirit Squire charges forward through multiple enemies, dealing damage and stunning them. 
• Lunar Lancer: Lunacy also spawns a dark shadow version of you that attacks enemies.
• Neon Ninja: Empowered Gem replaces the normal shuriken with a massive buzzsaw that passes through enemies, damaging every enemy in its path and applying Stasis Blade.
• Pirate Captain: First Mate becomes ARR-tillery, Allowing an additional First mate turret to be deployed
• Shadow Hunter: Basic attack becomes a rapid-fire stream of arrows. Dealing damage to a shadow-marked enemy consumes the mark and triggers an explosion.
• Tomb Raiser: Banshee's Boon no longer swaps your basic attack, instead it spawns a banshee minion that heals all of your other nearby minions.
• Revenant: Bulwark Bash loses its cooldown and applies a stacking damage-over-time effect to damaged enemies.
• Chloromancer: Spawning a Leafy Lasher or Blooming Pollinator has a chance to spawn a Green Gatling. It grows very quickly and unleashes a rapid stream of projectiles at enemies.

Empowered Gems for any class:

1. Explosive Epilogue. Killing enemies causes them to explode.
2. Volatile Velocity. Increases the shot speed of any projectile.  
3. Cubic Curtain. Avoiding damage grants a protective shield, absorbing the next attack and stunning the attacker.
4. Mired Mojo. Taking damage has a chance to spawn a ring of snaring projectiles, reducing enemy Movement Speed.      
5. Pyrodisc. Killing an enemy has a chance to generate a flaming disk that damages nearby enemies.  
6. Stunburst. Taking damage has a chance to spawn a ring of stunning projectiles, briefly disabling affected enemies.  
7. Spirit Surge. Taking damage has a chance to spawn a ring of projectiles, applying damage-over-time to enemies based on your max health.  
8. Stinging Curse. Dealing damage applies damage-over-time effects to enemies.

There are 9 slots and each is opened on some level. 

The First Normal Water Gem slot is unlocked at level 10.                                                                                                  The First Normal Air Gem slot is unlocked at level 13.                                                                                                       The First Normal Fire Gem slot is unlocked at level 17.                                                                                                      The Empowered Water Gem slot is unlocked at level 12.                                                                                                    The Empowered Air Gem slot is unlocked at level 15.                                                                                                       The Empowered Fire Gem slot is unlocked at level 20.                                                                                                       The Second Normal Water Gem slot is unlocked at level 23.                                                                                             The Second Normal Air Gem slot is unlocked at level 26.                                                                                                    The Second Normal Fire Gem slot is unlocked at level 30 (Max Level).

Each gems can have up to 3 stats in it: 

• Magic Damage (MD)
• Physical Damage (PD)
• Critical Hit (CH)
• Critical Damage (CD)
• Maximum Health (MH)
• Maximum Health% (MH%)
• Healt Regeneration (HR)

The best stat rolls for any class can be found in Builds section.

You can level up our gems with gem dust and flux to increase their stats or to add new stats. Gem dust can be obtained by deconstructing gems in loot collector, by opening any of gem boxes or Gem Booster Boxes. Gems can also break while being leveled up. But you will be able to repair cracked gems with Re-Gemerator which is also used to unsocket gems without losing them. It can be bought in store for 500 cubits.

You can also prevent cracking by using special boosters which can be obtained from Gem Booster Box, Chaos Chest, Daily Loot Chest, legendary tome or from weekly contests. There are 4 types of boosters(ALL THESE ARE UNTRADEABLE):

1. Jade Clover. Does not give any extra level up chance BUT prevents gem from cracking. That one is the easiest to get from contests.
2. Glittering Horseshoe. Also prevents gem from cracking and gives extra 50% level up chance. You won't find it difficult to get this one from contest either.
3. Lapis Luckbug. Prevents from cracking, gives 3x level up chance and 5x double level up chance. You will be able to recieve only 10 of these a week from contests.
4. Ninth Life. Gives 40x level up chance and 10x double level up chance. You can get one a week if you are rank 10000 or higher in Mastery Points section of weekly contests. 

I advice installing Karma tooltip mod. It will not only show your how much boxes you have to open to fill the karma bar on lockboxes but will also do the same thing with the gem karma bar.

I wanted to give some basic info about the gems to guys that are new to the game.Hope someone found this guide useful.