Splendor: can I use gem coins instead of development cards?
It's not explicit in the rules whether you're allowed to do that, but they say:


  if  a  player  has  2  blue  bonuses  and  wants  to  purchase  a
  card which costs 2 blue tokens and 1 green token, the player 
  must only spend 1 green token.


Use of the word must would suggest that the discount from the cards is mandatory.

Also, I can see no reason why you would want to use up coins when you don't need to. You can always take 3 coins (or 2 of the same colour if 4 or more are left). You only have to discard coins if you have more than 10 at the end of your turn:


  A  player  can  never  have  more  than  10  tokens  at  the  end  of
  their turn (including jokers). If this happens, they must return 
  tokens until they only have 10 left.


So in the example in your question you could carry on taking new coins and discard the unwanted emeralds at the end of your turn if you had more than 10 coins.