Gold costs 1 energy per block to mine. The amount of gold varies between 1 and 100. The gold can be used to forge or upgrade equipment. Level 53 is a very useful level to collect gold as it contains 150 gold and can be completed within 30 seconds. If you use a Gem Hammer, you can collect three times as much gold, resulting in 450 gold per completion of the level. Level 81 is better at higher levels, and easily done with a Gem Hammer or Diamond Pick (both x3 gold), plus a Mighty Camping (+1 bomb effect) or Mighty Rock (-50% tech cost) Backpack. Level 100 can get you 5025 gold, though it might cost you a life since you'll be ignoring everything else.
Collect all gold challenge:
If the challenge is to collect all gold, the last block will flicker.