Gems have 6 levels. The higher the level the greater the number of additional attributes. Diamond receives an increase for both stats at the same time.
The only way to increase the level of a Gem is to merge it with another one. There are two types of merges which we will depict in other articles.
Open the 'Inventory' tab,
Tap on a Gem you would like to merge,
Choose the 'Merge' option in the opened menu,
In the following screen, press the '+' button to use a regular Merge, or tap on a banner at the bottom to use a Super Merge,
Pick another Gem that will be merged,
Check the merge price at the bottom of the screen and press the 'Merge' button.
More detailed descriptions of merge types can be found in the articles: