If you don't want to just sit there clicking your mouse for gems, you can use this script by simply positioning your mouse over the craft button with ACtool (I use to use this all the time in Asheron's Call I:
// the script
SetActiveWindow Diablo III
Loop 200
LeftMouseDown
delay 200
LeftMouseUp
delay 3500
End
// What it does
The loop number is how many times you want it to run
The delay that I have at 3500 can be lowered to be a little more precise, but I was comfortable with it.
ACTool does allow you to set a mouse position (which I didn't add because I just don't feel like doing it atm.)
But, if you have your game in the same place, take a screenshot and find the mouse position using mspaint or whatever you use.
Then simply insert this after the loop statement:
MousePos X, Y
where X and Y is the mouse position of your craft button.
If any Dev has the time to improve it or turn it into a plugin, it's a real easy way to not have to sit and mouse click.
ACTool can be found here:
AC Tool Home Page