I've looked around the site for any thing like this but could not find anything about it.
So here goes hope I'm not blow in the wind.
All you do is make a normal gem. Lets say a Ruby (Tier 19) Flawless Royal Ruby then go to properties
go down to Rarity and change it to say 6 rare Then
go down to Quantity and make it any number you like lets say 10,000
now go to rare name and change to something you like
Remember SAVE after all changes are finished.
Now go to Mystic and change Old Affix to something that isn't the same as the affix you're going to use.
Then Change New Affix to what you want.
Your done. Now how to using them works.. In game Throw them all on the ground BUT 1
and place in your gear. Then pick them back up and do it again.
You can make multiple gems of varying kinds. For Example: Resist All Elements on a Diamond will give you
roughly 227 Resist All Elements per gem so x 3 = 681 Resist All Elements.
Make sure that you add more in your stacks of modded gems so they don't mix with any normal gems you may have in your stash box and or inventory.
Hope I did not miss anything..