Gems are the so-called "premium" currency that can be obtained during gameplay or by spending real money. You can use them to speed up the timer on a box or to get the required amount of gold needed to fuse parts. 

There are several ways to obtain gems: 

1. Open boxes

Tap on the box and open it. Opening the boxes will give you parts, gold, and a certain amount of gems. You can get the gems from the following boxes:
 
  • Super Box (Blue Box)
  • Legendary Box (Gold Box)

2. Complete Stages

You can obtain gems by completing Stages. Whenever you are promoted to the next Stage, you get Rating, Skill Points, Gold and Gems if your results were good enough.

3. Visit the store

Tap on the "+" icon next to the Gems icon to buy more Gems for real money. (You can find out how to disable in-app purchases in the Billing section)