If I have 10 gems, can I still reserve a card?
Yes, absolutely.

Very simply, if at the end of your turn you have more than ten tokens discard down to ten by returning your choice of tokens to their respective piles.  The only time when having more than ten tokens is relevant is very specifically at the end of your turn.

From the rulebook:


  Selecting tokens
  
  
    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. A player can return all or some of those they’ve just drawn. The tokens owned by a player must be visible by all players at all times.
    
    Reminder: players may not take 2 tokens of the same color if
    there are less than 4 tokens available of that color.