Once You Are At The Endgame And Mapworks Is Available. There Is A Guy That Sells 2-3 Gems At The Minehead. He Refreshes His Inventory If You Go To The Mapworks.
So The Easiest Way To Do This Is:
Buy The Gems, Warp To Mapworks, Warp Back To Minehead, Buy The 2-3 Gems Again.
This Does Require Some Cash To Do.
However You Can Use The Same Gem Again And Again And Again, The Game Doesn't Care.
500 Total Gems Socketed Nets You This Achievement.
Good Luck!