Blizzard have made quite a few changes to the Primordial Stone effects since their launch this week, and they detail them all below. 

Primordial Stones (Source)

Several hotfixes were made to Primordial Stones today. The intent here is to bring other Primordial Stones up to the competitive level where the Humming Arcane Stone is currently performing. Although, a portion of this level of performance can be attributed to a bug, even when fixed the Humming Arcane Stone was still the top performer. With these changes we’re hoping to make other combinations more viable in comparison, but not necessarily overtake it over from being the best option. In addition, area of effect stones have gotten a considerable increase to their output. These changes are listed below. Thank you for your feedback!

Cold Frost Stone effect increased by 12%.

Deluging Water Stone effect increased by 22%.

Desirous Blood Stone effect increased by 18.5%. Decreased trigger rate.

Echoing Thunder Stone effect increased by 12%.

Exuding Steam Stone effect increased by 25%.

Flame Licked Stone effect increased by 12%.

Freezing Ice Stone effect increased by 12%.

Gleaming Iron Stone effect increased by 12%.

Humming Arcane Stone – Fixed an issue that caused it to trigger more than intended.

Indomitable Earth Stone effect increased by 12%.

Pestilent Plague Stone effect increased by 72%.

Raging Magma Stone effect increased by 12%.

Searing Smokey Stone effect increased by 34%.

Shining Obsidian Stone effect increased by 12%.

Storm Infused Stone effect increased by 34%.

Wild Spirit Stone effect increased by 50%.