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.
Boon of the Hoarder 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.
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.