Dig as deep as you can. Deeper is hotter, and the gems are worth more.
You start with your bare hands. Dig stairs so you can jump back out. Sell what you find and upgrade at the shop on the surface. Your bag only holds so much. Walk into a wall to dig it; up into a ceiling digs up.
Each new layer can bring something new.
What you carry is in the bar at the bottom. Q and E step left and right (or press its number), Space uses it (hold it to take back a ladder or light you set). Tab opens the shop when you're at it, and your inventory anywhere else. Esc pauses. The pause page has every key, the sound and the display, plus Help, Rescue and Save game (C in your inventory opens it too).