This could be made into a very addicting game. The combo and the mining mechanic combined made it very fun to optimize how to mine blocks. This reminded when I played minesweeper, the feeling of recognizing patterns of mines (best combo routes in your case) and try my best to clear them as fast as I could. I thought it was a bug that I cannot upgrade my pickaxe, but turns out i recognized the wrong material, maybe different materials should use different colors for better distingtion? It was fun when I can one hit kill normal blocks. Overall the gameplay is solid and quite a unique twist on this genre. Good job!