The Dark Mirror of the Lapidary World

Obsidian is a stunning, naturally occurring volcanic glass. Born from rapidly cooling lava that didn't have enough time to form a crystalline structure, it comes in deep, mesmerizing blacks, swirling mahogany reds, and even flashing, iridescent rainbows.

For a beginner lapidary artist, obsidian looks incredibly inviting. It's cheap, abundant, and cuts like butter on a diamond saw. However, as soon as you attempt to polish it on a cabochon machine, obsidian reveals its true, frustrating nature.

It is notorious for developing deep, highly visible scratches just when you think you've achieved a perfect polish, and it frequently chips along the edges. Mastering obsidian requires strict discipline, impeccable cleanliness, and a specialized polishing progression.

1. The Fragility of Volcanic Glass

The core issue with obsidian is its physical structure. Because it is glass (amorphous silica) rather than a crystalline rock like agate, it has a Mohs hardness of only 5 to 5.5. It is soft and incredibly brittle, fracturing with razor-sharp conchoidal (shell-like) edges.

Beveling the Edges

The most common mistake when cutting an obsidian cabochon is leaving the bottom edge (the girdle) sharp. A sharp edge on obsidian is microscopically fragile. As you press the stone against the grinding wheels, tiny, invisible flakes of glass will chip off the sharp bottom edge.

These tiny glass chips get trapped between the stone and the wheel, acting like aggressive sandpaper, carving deep, looping scratches across the face of your cabochon.

The Fix: The very first thing you must do on the 80-grit wheel is grind a distinct, 45-degree bevel around the entire bottom edge of the stone. By rounding off the sharp bottom edge, you prevent it from chipping and contaminating your wheels later in the process.

2. The Threat of Cross-Contamination

Obsidian is so soft and dark that it acts like a mirror, highlighting every single microscopic imperfection. A scratch that would be completely invisible on a busy piece of Crazy Lace Agate will ruin a black obsidian cabochon.

If a single stray particle of coarse diamond grit (or a chip of glass) transfers from your 220-grit wheel to your 600-grit wheel, it will dig deep gouges into the obsidian.

The Fix: Impeccable Cleanliness.

  • Wash Everything: Between every single wheel change, stop the machine. Wash the stone thoroughly in a bowl of clean water. Wash your hands, paying special attention to your fingernails. Dry the stone with a paper towel and inspect it under a stark LED light.
  • Inspect for Rogue Scratches: Never move to a finer wheel until all scratches from the previous wheel are completely gone. If you see a deep scratch while on the 600-grit wheel, moving to the 1200-grit wheel will not fix it. You must go backward to the 220-grit wheel to grind the scratch out.

3. The Gentle Touch on Soft Wheels

Because obsidian is soft, diamond resin wheels (soft wheels) cut it extremely fast.

If you push obsidian hard into a 280-grit or 600-grit soft wheel, the diamonds will carve deep, uneven grooves into the glass. The stone will develop a bumpy, "orange peel" texture that is impossible to polish out.

The Fix: Use an incredibly light touch. Grip the stone lightly and let the spinning wheel do the work. Keep the stone moving in constant, sweeping figure-eight motions to ensure the dome remains perfectly smooth and symmetrical.

4. The Final Polish: Cerium Oxide

While you can technically take obsidian up to a 14,000 or 50,000-grit diamond paste, traditional lapidaries agree that the best, most reliable finish comes from a chemical-mechanical polish using Cerium Oxide.

  1. Stop at 1200 or 3000 Grit: Finish your diamond wheel progression so the stone is completely smooth with a hazy, pre-polish sheen.
  2. Prepare the Slurry: Mix cerium oxide powder with water until it reaches the consistency of milk or thin cream.
  3. The Buffing Wheel: Use a leather or thick felt buffing wheel. Ensure the wheel is damp (not soaking wet) and apply the cerium slurry.
  4. Generate Mild Friction: Press the obsidian firmly into the leather wheel. You want to generate a slight amount of heat through friction. The heat, combined with the cerium oxide, reacts chemically with the silica in the glass, melting the microscopic surface layer and flowing it flat.

Be extremely careful not to let the stone get too hot. If it burns your fingers, it is hot enough to shatter. Keep the wheel damp with a spray bottle.

With patience, strict cleanliness, and a good cerium buff, your obsidian cabochons will transform from dull grey rocks into deep, flawless black mirrors.