When the effect of invigorating gemstone procs, does it do so before or after taking damage?

The invigorating gemstone's rank 25 effect is to heal the player when a control-impairing effect hits. Does this heal go before or after taking damage from said effect?

Example:

• An elite pack has the frozen attribute.
• I stand in the area where frozen is going to hit while having this gem equipped and at rank 25
• The frozen goes off
• Do I get damage here and heal 20% of my life after?
• Or do I heal 20% of my life here and get damage from the frozen blast after?

I'm asking because I play hardcore, so things like this can really matter.