Use Mission Discoveries. Get a lvl 110 Treasure Hunting mission schematic from the Market, then run it. Provided the mission is not a total failure (which happens very rarely), you WILL get a few purples, usually 2 to 4. So far, that holds true for every mission skill I used.
This is incorrect. The wind crystal is a level 1 gem. The 115 treasure hunting mission discoveries return level 2 items. I have never seen a level 110 mission.
The two regular missions that I have managed to get wind crystals from are "Preying on the Meek" and "Sealed in Stupidity."