You earn Gems when you play the Mainland Mines.
There are three different types of Gems:
Green Gems
These Gems are used to buy Super Managers at the Super Managers beach on the world map.
Blue Gems
These Gems are used to upgrade the Super Managers you have bought.
Red Gems
These Gems are used to unlock Passive Skills for the Super Managers you have bought.
Instant Gems Items
Instant Green Gems Item -- from the shop, costs 1500 Super Cash.
Instant Blue Gems Item -- from the shop, costs 1500 Super Cash.
Instant Red Gems Item -- from the shop, costs 1500 Super Cash.
Power Crystals
Also check out the Power Crystals which are linked to Gems!