Raise Your Hand
Hold up your hand for 5 minutes or so, and wait as the tiny veins in your fingers drain of blood and slim down. This gets the diamond ring loose and lets you take it off without much hassle.
Soak in Ice Cold Water
Put your finger in ice cold water for 5 minutes or so. The low temperature will squeeze the blood vessels and make your finger effectively smaller. This will let you take the ring off easily.