When making matches of 4 or more Gems, matched Gems are transformed into Power Gems, also known as 'Big Gems'.
During battles, when making matches of 4 or more Gems, these Gems are transformed into a Power Gems. Starting at a basic Gem which is transformed into Power Gem I, and going up to Power Gem X.
This is also applied to any matches of 4 or more Skulls!
Here is an example of making a match of 4 Gems, which is then transformed into a Power Gem.
The more Gems that are matched and transformed into Power Gems, the more mana these give!