The furnace was built into the ground, lined with a rock sold as "Montana Red", sandy pale brown shot through with veins of dulled mica like dirty mirrors. We fed it charcoal and crushed dull green ore, kept the fire oxygen-free so that the sparks only flashed into existance inches above the ground. It smelled of mesquite charcoal, or baking bread on a stone, and when I spilled soda across the stone lid, the sugar carmelized instantly, rich and dark.
Of course, we made s'mores over the furnace full of molten rock. Moral imperative. It took about three seconds per marshmallow.
When the heat was over, we pulled back the covering rocks, crumbly and flaked from the heat, and found the white ashes thick with butterfly dreams. Knife-edged flakes of what had been sandy brown stone melted and recrystallized as translucent white marble, so thin you can see the heat of your fingers through them. Stuck askew in pairs and chains, strange pale three-dimensional calligraphy, held by drops and curls and spiralled antennae needles of glossy glass in black and green and gold and winding metallic ribbons of pure copper.
It was hard to hold them, shining winged things of marble and copper and glass, without expecting them to take off.
Edit: gfish's post (with photos) here, shadowblue's very marginally related post here.