The amount of favor stored in a gem is double the amount used to fill the gem
Upon examining a gem the ''approximate'' amount of favor stored can be found from the power emitedemitted.
1 - 50 It emits power.
Assume that the Priest has 50 Faith and a therefore a maximum of 50 Favor. The priest also has a gem of 25QL
If the priest had 30 favor and cast vessel on the 25QL gem their favor would drop to 0 and the gem would be completly completely filled with power, as the favor to completely fill a gem is the QL plus 5 Favor.
Now the priest, not wanting to wait to regain favor naturally uses the gem at once "You feel a rush of blood to your head as the power from the gem enters your body." the QL of the gem drops to almost 0 and the priests favor jumps up to 50. This is because the amount of favor used to fill the gem (not including the 5 used to cast) is doubled and given back to the priest