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.
Invigorating Gemstone 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.
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.