The Gem Forge is an add-on for Infinity Blade II, introduced in version 1.1. The Gem Forge allows the player to forge new gems by putting 3 gems in a 'Gem Forge' side menu, and waiting for said amount of time. For the first gem in a given Rebirth, the time needed is 30 minutes in real world (1 month in game time). Each subsequent forging increases the time needed by an additional 30 minutes. Gem Forge also requires an active Internet connection. Without it, the forging progress stops.
One can skip the process by paying an amount of gold, 2,500 per 30 minutes, scaling against how much time remains for the gem to be created. Another way to cut the time needed to forge gems is being reborn as it fast forwards time by 6 months in game time. Leaving gems to be forged and not speeding them up with gold generates no difference in the outcome of the gem.
The Gem Set found in Negative Rebirths allows a gem to be forged instantly after successfully completing an XP challenge.
Infinity Blade III
Gem forging reappears in Infinity Blade III, but is called gemcutting. Having Gemcutter is a prerequisite. The time can also be reduced by rebirth.
Gems/Guide:Infinity Blade III Gemcutting