The Golden Fever is the event where you have a chance to get golden and overall more valuable gems during the attacks. 

In order to activate the Golden Fever, you need to use Wizard Wands. You can obtain Wizard Wands once per day in the event tab. The current number of your Wands is displayed in the upper right corner. Wizard Wands are not obtainable through attacks and also cannot be purchased by orbs.

When you activate the Golden Fever in the event tab, you will see gems you can find during the attacks. While it is active, you will find more valuable gems in most of the dungeons you attack. You also can find gems that were not displayed in the event tab.

The time until the end of activated Golden Fever is displayed in the event tab and the upper right corner during the attacks.

While the Golden Fever is still active, you can use Wizard Wand to prolong its effect.

After the activation of the Golden Fever ends, everything returns to normal, and the chance to find golden gems and more valuable gems returns to usual. 

Before the next activation, the Golden Fever has to recharge. You can decrease recharge time by using Wizard Wands.