As others have stated, it is the same whether you spend 1 gem each on five separate cool downs or 5 gems on one cool down. 1 gem is always equal to 2 hours of cool down except on Thorsday when it is 8 hours worth of cool down. So as long as you only spend gems on cool downs that were initiated during Thorsday, you are getting the most value for your gem. Spending at each 2 hour increment maximizes the gem's value. ie. don't spend 2 gems to reset a CD with 2:05 left, wait the 6 minutes and spend 1 gem at 1:59.