When a person pays cash for gems, gems are created and put into the economy. That person has three choices as to what they want to do with those gems: spend them in the gemstore, save them, or convert them to gold. Very straightforward.
When a person pays in-game gold for gems, gems are not created, as they are only created by paying cash for them (or dev commands, I’m sure).
Going between gold and gems, or vice versa, results in a 15% charge just like the TP.
The third option for gem acquisition is from rewards. Achievements provide 400 gems in the 5k chests.