Very interesting gameplay loop! The controls feel responsive and the mining feels good! More levels would be nice, maybe having the first level a lot smaller, and every new portal you advance to a bigger level with some changes to the stone/dirt colors to make it feel like a new level; maybe even add in more valuable ores each level! As currently with the random placement of the portal, it seems like a good run where you find a portal is more random than rewarding. Regardless, that was a fairly good game and I very much enjoyed it!