This gem gives a chance to cause any killed enemy (even if killed by a pet or a minion) to drop several large piles of gold in addition to their normal loot. The explosion does not cause any damage. Upgrading the gem increases the chance to proc; as of patch 2.4.0, the gem cannot be leveled past level 50 (when the chances reach 100%).
At rank 25, it also gives a brief movement speed boost after picking up gold.
This gem will have no effect in Greater Rifts. It will also not trigger off of kills made by anyone other than the Nephalem hero who uses this gem, even their own followers. It is also the only gem that gives no real combat effectiveness by itself. The massive quantities of gold offered, however, do prove very potent when coupled with items like Goldwrap and the Custerian Wristguards.
The Boon of the Hoarder is one of the few gems that actually have a limit of effective ranking.
Boon of the Hoarder
"Rumors speak of a greedy Baroness who sends forth minions to collect treasure, equipped with gems to aid in gathering ever more. Boon of the Hoarder is said to be one of these precious gems." — Abd al-Hazir