I did some testing to try to determine the actual proc rate of
[Insightful Earthstorm Diamond]. While Wowhead says it's 5%, I have never observed a proc rate that high in a raid.
I cast Healing Wave Rank 2 for 7.5 hours. I cast it 17,428 times and the meta gem procced 622 times, giving us a proc rate of 3.57%.
Here's the WWS:
Skyhoof - WWS
I used AutoHotKey to spam the spell for so long.