Bringing Ancient Forests Back to Life

Petrified wood is one of the most popular and rewarding materials for lapidary artists to work with. Found on nearly every continent, these stones offer a profound connection to the deep past, allowing you to hold a piece of a tree that may have stood in the shadow of dinosaurs over 200 million years ago.

Because the organic cellular structure of the wood has been perfectly replaced by minerals (a process called permineralization), petrified wood can be cut and polished to a mirror finish, revealing breathtaking wood grain, growth rings, and vibrant colors caused by trace metals.

However, despite essentially being a form of quartz or agate, petrified wood presents unique challenges on the lapidary bench.

1. Understanding the Material

To cut petrified wood effectively, you must understand what you are cutting. You are no longer cutting cellulose and sap; you are cutting a solid rock.

In most high-quality petrified wood (like the famous logs from the Petrified Forest in Arizona), the replacing mineral is silica (silicon dioxide). This means the wood has a Mohs hardness of around 7, similar to agate or jasper.

The vibrant colors in petrified wood are not from the original tree species, but from heavy metals in the groundwater that permeated the wood during fossilization:

  • Red and Brown: Iron oxide
  • Green and Blue: Copper or Cobalt
  • Black: Carbon or Manganese oxide

2. Slicing and Orienting the Cut

The beauty of a petrified wood cabochon depends entirely on how you slice the rough log on your slab saw.

  • Cross-Cut (Transverse): Slicing directly across the log, perpendicular to the trunk. This reveals the concentric growth rings and the heartwood. It is perfect for large display specimens and dramatic, bulls-eye patterned cabochons.
  • Longitudinal Cut (Lengthwise): Slicing parallel to the trunk. This reveals long, sweeping, straight grain lines. It creates beautiful, subtle cabochons that mimic the look of traditional lumber boards.

When setting up your slab saw, always inspect the outside of the log for deep, weathered fractures. Petrified wood is notorious for containing hidden cracks that developed over millions of years of geological pressure.

3. The Challenge of Undercutting

The most frustrating issue lapidarists face when grinding petrified wood is undercutting.

Because the fossilization process is not always uniform, a single slab of wood might contain hard quartz veins directly next to softer areas of calcite, opal, or compacted volcanic ash.

When you press the stone against a soft resin polishing wheel, the wheel compresses. The abrasive diamond grit easily carves out the softer areas of the stone but barely scratches the hard quartz veins. After a few minutes of polishing, you will notice that the surface of your cabochon is no longer a smooth dome; it is bumpy and rippled. The hard veins stand up like mountain ridges, while the soft areas are carved out into valleys.

How to Prevent Undercutting

  1. Use Hard Wheels Longer: Do the majority of your dome shaping on your hard steel diamond wheels (80 and 220 grit). Hard wheels do not compress, so they grind the hard and soft areas at the exact same rate, keeping the surface perfectly flat.
  2. Light Pressure on Soft Wheels: When you move to your soft resin polishing wheels (280, 600, 1200 grit), use an extremely light, feathered touch. Let the diamonds do the work. If you push hard, the resin backing will compress into the softer areas of the wood and gouge them out.
  3. Keep the Stone Moving: Never hold the stone still on the wheel. Constantly rotate and sweep the stone to distribute the grinding friction evenly across the entire surface.

4. Achieving a Mirror Polish

Because petrified wood is primarily silica, it polishes beautifully using standard lapidary progressions.

After you have progressed through your 1200 or 3000 grit diamond resin wheels, the stone will look shiny, but it may lack that final, liquid-glass gleam.

To achieve a true mirror polish, you need to use a polishing compound. Cerium Oxide is the absolute best polish for silica-based stones like petrified wood.

  • Mix cerium oxide powder with water to create a milky slurry.
  • Apply the slurry to a leather buffing wheel or a felt polishing pad spinning at a low RPM.
  • Press the petrified wood firmly against the buffing wheel, generating a slight amount of heat (friction).
  • Keep the wheel damp—if it dries out, the heat builds up rapidly and can shatter the stone.

The friction and the chemical properties of the cerium oxide will literally melt the microscopic top layer of the silica, flowing it over any microscopic scratches to create a flawless, wet-look finish.

Cutting petrified wood requires a keen eye to read the grain and a gentle touch on the polishing wheels, but the reward is a piece of ancient natural history reborn as a modern jewel.