You can combine two gems for a chance to get a gem of a higher level. It might take several attempts since there's always a 1%-20% probability of receiving a gem of the same level. In rare cases, you can get a gem of a lower level. Sometimes you get lucky and will get a Diamond.
All levels can be merged among themselves. Here's the table to simplify and show the simplest way to get a gem a level higher:
The cost of a single merge attempt depends on the level of both gems and the amount of gem stats (one — Alexandrite, Sapphire, Ruby, Limonite; two — Diamond). The cost per merge starts from 20 KCAL and also depends on KCAL's current market price. That way, we can ensure a stable price in fiat equivalent and provide additional support for the game economy.