Many Upgrades have Special Upgrades that can power up that Upgrade. Some add more gems per second for each instance of a purchased upgrade and some even DOUBLE the gems per second being generated by that upgrade.
For example: If you purchase 3 of Drakenor’s “Quest For Coins” upgrades (each worth 3 gems per second) then Drakenor will be earning 9 gems per second. If you purchase “Quest For Coins” Special Upgrade called “Coin Polish” it will add a X2 upgrade to each “Quest for Coins” that you have purchased, so Drakenor will then be earning 18 gems per second.
Most Special Upgrades can be purchased with gems but some very powerful Special Upgrades can only be purchased with Dragon keys.