Upgrade rank grants: +1 second buff duration.

Gain 30% increased damage for 20 seconds after killing an elite pack.

The following are the most current details about Legendary Gems.

A major revision was created on Jwith new or upgraded stats for almost every gem. While the Legendary Gems are still undergoing development on the PTR, their stats and bonuses are changing constantly. The main difference in Diablo 3 is that these gems can only be socketed in jewelry, and the way the gems can be upgraded to improve their functions over time. While the developers are calling them “legendary gems” these socketables have nothing in common with regular gems in stats or appearance, and are more analogous to the Rainbow Facet unique jewels of Diablo 2. The gems add special bonuses when socketed in rings and amulets (only on Characters and not on Followers), and can be upgraded in power via Urshi, the NPC who appears after Greater Rifts are cleared. They will be added to Reaper of Souls in Patch 2.1, and are currently undergoing testing on the PTR.

Legendary Gems were first officially revealed in the Patch 2.1 preview blog back in June of this year.

Diablo 3 remove legendary gem