20,000 gold is roughly 91% of all gold value found in the mine. While you can certainly focus on this earlier, it becomes much easier with the Ore Detector from Vectron. Once you get to the bottom of Vectron, start there and work your way back up to start with the most money. You'll effectively need to clear out Vectron and the Old World for this; the money in Archaea is pretty minimal when you have access to the rest.
Gems can be destroyed by falling rocks and magnetic crushers, though you'd have to really try for it to destroy 2,000 gold's worth. Similarly, you only need to collect, so spend what you need! Finding all secrets within caves will definitely help matters as well.