As I imagine, adding gems to the pickaxe decreases the durability of the steel as you are degrading the structural integrity of the pick. sort of like drilling holes in a wooden beam. I assume gems increase enchanting level of the tool, though you can’t see that from the tooltip.
Greg is it possible to show the corresponding durability of the complete pick on the gem pickaxe head? I see what psusui is saying, the gem tipped pickaxe head durability is misleading as it shows the gem durability rather than the pickaxe head durability.