Lapidary

Grind a found stone for weeks until it shines as if it were wet

The cheapest start is a rock tumbler: stones and coarse grit go in, you step down to finer grit each week, and four or five weeks later they come out polished, with little hands-on work. Beyond that is cabbing, grinding stones into domed cabochons on wheels at a club or studio. Tumblers run nonstop and noisily, and one leftover coarse grain scratches the whole batch.