Obsidian is a cryptocrystalline mineral with a reputation for both beauty and for being difficult to polish. Unlike most minerals, the brittle nature of obsidian makes it difficult to use in rock tumblers. Even when the center of the tumbler is filled with corn syrup and plastic beads, the stones can chip and fracture against each other. Most lapidary artists choose instead to polish their finer pieces by hand, first with fine grain sandpaper, then with cerium or aluminum oxide polish.
Cut the 600, 1200 and 3000 grit sandpaper into 1-inch squares. Each square will last for roughly five minutes of polishing.
Buff the edges of the stone with 600 grit sandpaper, taking care to give all parts of the stone equal coverage. Use circular motions to avoid scouring gouges into the surface of the rock. The 600 grit sandpaper will remove the larger imperfections in the surface of the stone. This process can take two to three hours. When the surface of the stone ceases to improve with further sanding, progress to the next step.
Repeat Step 2 using 1200 grit sandpaper. Once again, when the surface stops responding to additional polishing, move on to the 3000 grit sandpaper. After finishing sanding with 3000 grit sandpaper, the surface of the stone will not show any evidence of scratching to the naked eye or under a ten power magnifying loupe. At this stage, the stone will begin to reflect light.
Add the final polish to the stone by placing cerium or aluminum oxide polishing compound on a lapidary polishing cloth. Buff the surface of the stone with the polishing compound until it achieves the desired shine. To achieve a higher degree of shine, apply .2 micron polishing compound to your lapidary cloth and perform an additional polish.