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.
Red Soul Shard 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.
Legendary Gem in Diablo 3
Legendary Gem Creation:
If you need extra Legendary Gems to level up, you can make the Rift Guardian in any Greater Rift drop a new Rank 0 Gem by removing all the Legendary Gems with the same name from your Equipment, Inventory and Stash.
The safest way to do that is to put them into the Inventory of different character.