The Greater Rift Keystone drop rate is 100% for T6 and maybe 75% for T1, lower for lower difficulties and chance of 2 dropping in higher difficulties. Is there any empirical data that confirms rates?
Edit: drop rates were changed in a hotfix on Sept 3, 2014:
The drop rate for Key of Trials has been significantly increased (9/3)
Edit: difficulty requirement was removed in patch 2.3 on Aug 25, 2015:
Generic Greater Rift Keystones now drop and can drop at any difficulty level
Confirmed level 70 required.
I'd still like to know approximate drop rates on all difficulties if anyone has data/insight.