Legendary Gem Mechanics

Mechanics
The player gains 20% multiplicative damage for 30 seconds after killing an Elite pack. In case of Rare (Yellow) Elites you also need to kill all the minions to receive this bonus. Rift Guardians, Act Bosses, Treasure Goblins and unique purple Monsters also give you the buff when slain.

You can level this Legendary Gem all the way up to rank 150. Every upgrade will increase the duration of the multiplicative damage buff by 1 second up to a maximum of 180 seconds. The secondary effect increases damage against Elites by 15%, which is additive to other Elite Damage sources like from The Furnace and multiplicative to base damage. Furthermore the secondary effect reduces damage taken from Elites by 15%. This bonus is multiplicative to other Elite damage reduction sources like Blackthorne's Battlegear (3) Bonus.

Best Use
Offensive in speed farming
Most commonly used in speed farming content. This includes Greater Rift Keystone farming and lower tier speed Greater Rifts. This is the first Legendary Gem that drops in a fresh Season and is widely used in the beginning as it is very powerful even at a low ranks.

Mechanics
Each attack you perform against an enemy increases the damage it takes from your attacks by 0.80%. You can level this Legendary Gem all the way up to rank 150. Every upgrade will increase the damage gain by 0.01% up to a maximum of 2.30% per stack. The stacks are additive with themselves and multiplicative with all other damage bonuses. The secondary effect increases damage against Rift Guardians and Act Bosses by 25% multiplicatively.

• First enemy hit with a Skill that can proc Bane of the Stricken receives a stack of debuff that increases all damage you do to that particular enemy.
• After applying a debuff to an enemy, Bane of the Stricken goes on Internal Cooldown equal to "0.9/Skill APS", where "Skill" is the Skill that was used to apply the debuff.
• The above formula is only true for all non-channeling skills and Whirlwind, all other channeling skills make Bane of the Stricken go on longer cooldown than their attack rate would imply.
• Generally only skills with a proc coefficient can proc Bane of the Stricken, meaning that Pets and item procs will give you no stacks. However, this is not a hard rule and there are exceptions.
• Normal enemies and Elites lose all Bane of the Stricken stacks when they leave combat. Rift Guardians never lose stacks even if you as the Player die.
• For a deeper look into Stricken mechanics and RGKs, read this guide from Chewingnom.

Best Use
The only way to kill a Rift Guardian at higher Greater Rifts
Nearly every Solo Push build and every Rift Guardian Killer uses this gem to kill the Rift Guardian in an adequate amount of time.

Mechanics
This is the most commonly used damage Legendary Gem, increasing your damage by 15% multiplicatively against enemies under the effect of control-impairing effects. You can level this Gem all the way up to rank 150. Every upgrade will increase the damage gain by 0.3% up to a maximum of 60% total. Control-impairing effects are: Fear, Stun, Freeze, Chill, Knockback, Immobilize and Slow including the Rank 25 effect of this Gem, making it proc itself in a 15 yard radius.

Best Use
The DPS Gem
Used in basically every damage dealing build. This is one of the most powerful and commonly used Gems in the game! Ranged Cold-based builds like the Frozen Orb Wizard have an easy time proccing this Gem as many Cold skills momentary apply Chill. In Group gameplay this Gem is usually proc'd by the supports with skills like Furious Charge, Crippling Wave Tsunami or Multishot Wind Chill.

Mechanics
When you kill a monster there is a 25% base chance for the monster to drop multiple Gold piles in an explosion. For each rank up of this Gem the chance increases by 1.5% and caps out at rank 50 with 100% chance. The secondary effect gives 30% Movement Speed for 3 seconds after picking up Gold. This bonus can exceed the 25% base Movement Speed cap.

Best Use
For Torment Farming
This has nice synergy with Goldwrap when farming Greater Rift Keystones for insane toughness values. The additional 30% Movement Speed increase also helps farm efficiently. In Greater Rifts however, none of the monsters except the Rift Guardian can drop Gold, hence it should never be used there.

Mechanics
16,000 additional Thorns damage is added to your character. You can level this Legendary Gem all the way up to rank 150. Every upgrade will increase this bonus by another 800 up to a maximum of 136,000 Thorns damage in total. The secondary effect taunts the first enemy hit with a primary skill to attack you for 2 seconds.

This Gem's enormous Thorns amount significantly reduces the importance of Thorns rolls on your items as it is so high in comparison. This generally makes Thorns builds a bit easier to gear. The secondary taunt effect is rather annoying for most builds as it slowly adds crowd control resistance to monsters. For more informations about Thorns, read this guide written by Chewingnom!

Best Use
Mandatory in Thorns builds, useless everywhere else
Awesome for all Thorns related build like the Akkhan Invoker Bombardment Crusader.

Mechanics
All pets gain 15% multiplicative damage. You can level this Gem all the way up to rank 150. Every upgrade will increase the damage gain by another 0.3% up to a maximum of 60% total. With the secondary rank 25 effect reducing pet damage taken by 90% they become almost immortal.

Best Use
Used in all pet builds
This is the signature Gem for Builds like Hydra Wizard or LoD Skeletal Mage Necromancer because of its large and unconditional damage bonus!

Mechanics
This Legendary Gem reduces all non-Physical damage taken by 10%. You can only level this Gem to rank 100. Every upgrade will increase the damage reduction by another 0.5% up to a maximum of 60% total. The rank 25 bonus is one of the strongest in the game. It increases your resistance to Cold, Fire, Lightning, Poison and Arcane by 75% while below 50% Life. This secondary alone is about an additional 40% total damage reduction to non-Physical damage. Alltogether these 2 effects combine for approximately 75% total non-Physical damage reduction. For a deeper look into damage reduction mechanics, read this post written by Northwar.

Best Use
The #1 toughness Gem
Used by Supports and Damager Dealers alike. Can be super useful especially at lower Paragon levels early on in the Season, but is mostly outclassed in the endgame by offensive Gems or other supportive choices.

Mechanics
This is the only legendary Legendary Gem that is socketed into the Weapon slot! Every monster that is killed grants 500 additional experience points. You can level this Gem all the way up to rank 150. Every upgrade will increase the experience points gained by 50 up to a maximum of 8,000 total points per monster. This effect scales with Torment difficulty but not with Greater Rift levels. If you want to find out more in-depth experience scaling mechanics about Experience per Kill and more check out this post. The rank 25 effect allows you to use this Gem and the weapon it is socketed into at any character level.

Best Use
For leveling only
This Gem is super useful to level a new character with its rank 25 effect or boost a friend since the experience bonus is shared in the party. This Gem is also a part of Work of Cathan Kanai's Cube recipe to further allow low level characters equip high level items. For Paragon leveling however this bonus experience is irrelevant as it does not scale with the Greater Rift experience multiplier.

Mechanics
This Gem applies a Poison Damage over Time effect for 2000% weapon damage over 10 seconds onto enemies when attacking them. You can level this Gem all the way up to rank 150. Every upgrade will increase the Poison damage by another 50% weapon damage up to a maximum of 9,500% total.

The rank 25 secondary bonus is the reason this Gem is played in every Support build in the game: All enemies you poison take 10% additive increased damage from all sources (including your party members) and also deal 10% less damage. Only the additive damage increase effects stacks with multiple Gem of Efficacious Toxin in the party!

Best Use
The support Gem
Used in most support builds for the awesome party benefits. Has not been used in an offensive way in ages as the base DoT damage is way too low. Leveling this gem past rank 25 is not recommended because DoT damage is completely irrelevant. As a reminder: You cannot give this to a follower and get the benefit like mentioned in the introduction!

Mechanics
Hitting an enemy gives you 1% Attack Speed and 0.5% Dodge Chance for 4 seconds, stacking up to 15 times. You can level this Gem all the way up to rank 150. Every upgrade will increase the Dodge Chance by another 0.01% weapon damage up to a maximum of 2% per stack total.

On top of that the rank 25 bonus increases your Cooldown Reduction by 1% per stack. On Rank 150 with 15 stacks active this Gem will give you at 15% Increased Attack Speed, 30% Dodge Chance and 15% Cooldown Reduction. Generally only skills with a proc coefficient can give you a Gogok of Swiftness stack, meaning that Pets and item procs will give you no stacks. However, this is not a hard rule and there are exceptions.

Best Use
Mostly a Support Gem but also played on some Damage Builds
One of the best support Gems in the game but also commonly used on Cooldown dependent DPS builds like the Vyr-Chantodo Archon Wizard.

Mechanics
Your Cold skills now always Chill monsters and slow their Movement Speed by an additional 5.0% for 3 seconds. You can only level this Gem to rank 50. Every upgrade will increase the slow effect by 0.4% up to a maximum of 25% total.

This Gem however, is mainly used for its secondary effect: 10% additional Critical Hit Chance debuff against Chilled or Frozen enemies. This effect procs itself with the primary Bonus as long as you use any Cold skill. This effect does not work for most Damage over Time (DoT) skills (e.g. Rend, Haunt) as their damage is calculated directly from the player's sheet Critical Hit Chance and Critical Hit Damage. This effect does not stack with other players wearing this Gem.

Best Use
Only used on Supports
Very common support Gem due to the huge secondary effect. This effect usually boosts the Damage Dealer by around one Greater Rift Tier. While this Gem is almost never used in any solo build, it's part of every META group.

Mechanics
Each successful hit done by the player increases healing by 1% multiplicatively for 5 seconds. This effect can be stacked up to 10 times and each new stack refreshes the duration of all stacks. You can level this Gem all the way up to rank 150. Every upgrade will increase the heal effect by 0.02% up to a total of 4%.

The secondary rank 25 effect grants you immunity to all control-impairing effects and is comparable to effects from skills like Ignore Pain or Inner Temple of Protection. Most enemies will interrupt your channels (such as Town Portal) even with this Gem equipped. To learn more about Crowd Control mechanics, take a look to this post written by Chewingnom!

Best Use
Mostly for supports
Not used a lot. Has some implications due to the rank 25 immunity effect for Support or Solo Push, especially in combination with the Stone Gauntlets.

Mechanics
While you have no set bonuses equipped (you can get this bonus if set items are equipped as long as no 2-piece or any other set bonus is active) every legendary item increases your damage dealt by 3.75% multiplicatively and reduces all damage taken by 2%. The multiplicative damage effect increases by 3.75% per level to a maximum of 375% at max Gem rank 99.

The secondary effect, unlocked at rank 25, doubles this bonus for a total of 750% damage and 4% damage reduction per Ancient or Primal item. As players in Diablo III can wear a maximum of 13 items, the maximum total bonus from this Gem is 13 x 750% = 9750% damage and 13 x 4% = 52% damage reduction. This effect is very similar to the Legacy of Nightmares set, but it has the benefit of granting you half the bonus for non-ancient items and it only costs you one Gem instead of two Rings. With this Gem equipped it is okay to miss out on a few ancient items as you only lose around 4% damage and 2% toughness if you are missing 1 Ancient Item.

Best Use
Widely used on damage dealing builds, especially on Necromancers
One of the best Gems in the entire game. This Gem and build style is so popular that people refer to them as "LoD" builds for Legacy of Dreams. So if you hear that saying in the Diablo community, they're talking about a no-set build based solely on this Gem!

Makes for a very strong Season start as you have a super high chance of dropping this Gem very early on (usually right after Bane of the Powerful). Furthermore this Gem fuels the strongest speedfarming group build in the game the LoD Skeletal Mage "Rat" Necromancer as it allows Necromancers to use extremely powerful ring choices like Krysbin's Sentence and Convention of Elements with only 3 Elements.

Mechanics
Gives a 15% chance on hit to smite a nearby enemy for 3,000% Holy damage and heal yourself for 3% of your maximum Life. You can level this Gem all the way up to rank 150. Every upgrade will increase the Holy damage by another 60% weapon damage up to a maximum of 12,000% total.

On top of that the rank 25 effect separately smites nearby enemies every 3 seconds. The initial 15% chance to smite is further reduced by proc coefficients of the used skill. It does not trigger from your pet attacks and hits a random enemy within a 25 yard radius upon proc.

Best Use
Never used in a real build
Unfortunately this Gem is way too weak at the moment and has no good use. It used to be pretty decent back in the days with Necromancer Inarius builds but simply lacks the damage for current META builds.

Mechanics
This Legendary Gem increases Life Regeneration by 10,000 per Second. You can level this Gem all the way up to rank 150. Every upgrade increases the Life Regeneration by 1,000 per Second up to a total of 160,000 per second. On top of that the secondary rank 25 grants you a shield for 200% of your total sheet Life per Second after not taking any damage (including damage absorbed by shields) for 4 seconds. Life per Second effect from skills, passives and other items also count towards this shield effect.

Best Use
Very useful synergy with Squirt's Necklace
Can be used for Speed runs to protect Squirt's Necklace's buff. Has a nice synergy with Draw Life, Mantra of Healing and other Life per Second increasing buffs. Sometimes used on supports for sustain.

Mechanics
35% of all damage taken is instead staggered and dealt to you over 3 seconds. You can level this Gem all the way up to rank 150. Every upgrade will increase the duration by 0.1 seconds up to a total of 18 seconds at the highest rank. On top of that the secondary rank 25 provides you a 20% chance to clear all previously staggered damage. The staggered damage also applies to shields consuming your shield slowly if not cleared by the rank 25 effect.

Best Use
Never used
This Gem is a full defensive one and never used as there are better defensive legendary Gem alternatives like Esoteric Alteration.

Mechanics
Gain 10% melee damage reduction. You can level this Gem up to rank 100. Every upgrade increases melee damage reduction by 0.5% up to a total of 60% at rank 100. On top of that the secondary rank 25 provides the ability to move unhindered through enemies while you are below 50% life. This effect is the same as Illusory Boots.

Best Use
Super rare in builds that get stuck a lot or have no great toughness sources
Nowadays this Gem is almost never played as we have plenty other skills like Teleport or Blood Rush and many more to move past enemies. Furthermore melee reduction does not reduce most damage in the game and overall provide a very inconsistent toughness buff.

Mechanics
Critical hits cause the enemy to bleed for 2,500% Physical damage over 3 seconds. You can level this Gem all the way up to rank 150. Every upgrade will increase the Physical damage by another 50% weapon damage up to a maximum of 10,000% total.

The rank 25 bonus is the true powerhouse of this Gem as it provides 3% increased additive Attack Speed for each bleeding enemy within a 20 yard radius. "Bleeding" in this case does not refer to the gem's primary effect, enemies affected by any Physical Damage over Time effect (e.g. Chance to Bleed on Hit, Rend, Locust Swarm Cloud of Insects, Exploding Palm, Consecration Bed of Nails) will still give you the bonus Attack Speed.

Best Use
Widely used on Attack Speed dependent builds and group trash killers
This is a signature Gem for a lot of Pets builds as Pets scale pretty well with Increased Attack Speed. The Mundunugu Spirit Barrage Witch Doctor for instance is a build that gets tremendous value from this Gem.

Mechanics
This Gem can only be found in the January Anniversary Darkening of Tristram Event. Periodically struggle for control, unleashing a Ring of Fire that deals 12,500% Fire Damage to enemies that it passes through. You can level this Gem all the way up to rank 150. Every upgrade will increase the Fire damage by another 50% weapon damage up to a maximum of 20,000% total.

The rank 25 bonus reduces your cooldown on skills by 75% and removes your resource costs completely after you level up for 30 seconds. The Cooldown Reduction part is displayed on your in-game stats and works with Captain Crimson's Trimmings damage increase. The Resource Cost reduction part however does not work as a toughness increase and is not displayed in the in-game stats!

This Gem can be used at any Character Level and is the only legendary Gem that is socketed into a Helm. The Fire damage surrounds you in a circle bursting to all sides and can crit. It procs every 30 seconds. It is indicated by a 2 second self stun. This stun duration can be influenced and avoided with crowd control reduction and immunity from skills like Ignore Pain.

Best Use
Only used during leveling but even here the real important gem to use is Gem of Ease
This unique Gem has been used for some funky builds in the past when the duration of the buff from secondary effect stacked. This has been fixed and that leaves this Gem rather uninteresting at level 70.

Mechanics
Increases the damage of primary skills by 25% multiplicatively. Primary skills are skills that do not cost Resource to cast and are normally rather weak in terms of damage output. They usually generate Resource. Wizard and Witch Doctor have Signature Spells like Poison Dart or Magic Missile that do not generate resource, they are considered primary skills hence buffed by this gem.

You can level this Gem all the way up to rank 150. Every upgrade will increase damage by 0.5% up to a maximum of 100% total. The rank 25 bonus grants 4% healing of your maximum health whenever you hit with a primary skill. It can only heal you once per cast even if you hit multiple enemies.

Best Use
The Generator Gem
Used in all damage dealing builds that rely and stack primary Skill multipliers like the Frenzy Barb or Zunimassa Poison Dart Witch Doctor. The insane healing effect on top of the multiplicative damage increase makes this Gem mandatory for these builds!

Mechanics
Gain 2% additive damage for 1.5 seconds when you spend resources with a channeling skill. This effect stacks up to 10 times. You can level this Gem all the way up to rank 150. Every upgrade will increase the additive damage by another 0.04% per stack up to a maximum of 8% total. Furthermore the rank 25 bonus increases your Armor by 2% for each stack. At max rank and stacks this Gem provides a whopping 80% additive damage increase as well as a 20% Armor buff.

Best Use
Very strong for channeling builds
This Gem grants offensive and defensive buffs alike and is used by almost every build that relies on channeling Skills like the WW Rend Barbarian or the PoJ Tempest Rush Monk. In groups however, its damage value is generally significantly decreased as this gem grants additive damage and supports already bring quite a lot of additive damage buffs to the party. For a better understanding about damage multipliers, read this in-depth article from Chewingnom!

Mechanics
Gives a 15% chance on hit to gain 4 second buff that deals 1,250% Lightning damage every second to enemies within 30 yards. This effect sounds similar to the Conduit Pylon Lightning. You can level this Gem all the way up to rank 150. Every upgrade will increase the damage by 25% per stack up to a maximum of 5,000% total. The rank 25 bonus increases your Movement Speed by 25% when the Lightning buff is active. The initial 15% proc chance is further reduced by proc coefficients of the used skill and does not trigger by attacks from your pets.

Best Use
For speed farming
This Gem is mainly used for the rank 25 Movement Speed increase in Greater Rift Keystone and Bounty farming. The actual damage from the Lightning effect is meaningless, but it has nice synergy with Warzechian Armguards, as Lightning effect can destroy breakable objects, constantly triggering Warzechian Armguards Movement Speed without making you stop to use an attack.

Mechanics
Increases damage by 4% multiplicatively for every 10 yards between the player (pet damage is also based on the player-monster distance) and the enemy hit. Maximum 20% at 50 yards. You can level this Gem all the way up to rank 150. Every upgrade will increase the damage by another 0.08% per stack up to a maximum of 80% total.

The tooltip ismisleading as the first damage bonus is already active when you are right on top of the monster. That also means the maximum damage bonus of 80% at rank 150 is already reached at 40 yards. The damage goes up in 10 yard steps at 10, 20, 30 and 40 yards, meaning there is no additional damage increase at 4 or 5 yards etc..

The secondary effect has a 20% chance to stun the enemy for 1 second when you hit them. This chance is affected by the skill's proc coefficient. Overall this effect is not very useful and can even have a negative impact as it adds random crowd control immunity to monsters when proc'd. This can be particular annoying on Rift Guardian fights where some boss killer builds rely on controlling the boss to deal optimal damage like the AoV Heaven's Fury Crusader with Bracer of Fury.

Best Use
One of the best offensive Gems
Used by almost all ranged builds and even on some melee builds, especially in speed runs. Currently this is one of the best choices for the strongest build in the game the Bone Spear Necromancer.