The Physics of Volcanic Glass

Obsidian forms when silica-rich lava cools so rapidly that crystals do not have time to grow. The result is an amorphous mineraloid—in essence, natural volcanic glass.

Because it lacks internal cleavage planes or crystalline structure, obsidian breaks via conchoidal fracture.

This means impact energy spreads outward in curved, shell-like waves, creating razor-sharp edges. While excellent for ancient arrowheads, it makes obsidian notoriously difficult to cut without chipping the edges.

Here is how to cut obsidian without shattering your preforms.

1. Use the Right Blade and Saw Setup

The most common reason for obsidian shattering on a trim saw is blade wobble or thick, aggressive diamond rims.

  • Blade Choice: Use a continuous-rim diamond blade. Ensure the blade is thin (0.015" to 0.025" thickness for a 6-inch saw). Thick, aggressive blades create too much vibration.
  • Flange Check: Check your saw flanges for warp or play. Even a microscopic wobble in the shaft will hammer the obsidian, leading to massive chips along the cut line.

2. Feed Rate and Pressure Rules

When cutting agate, you can apply moderate pressure to speed up the cut. With obsidian, you must modify your technique:

[Too Much Pressure] ---> High Vibration ---> Conchoidal Micro-Fractures ---> Shattered Edge
[Gentle, Steady Feed] -> Smooth Abrasion --> Clean Polish-Ready Edge

Pro Techniques:

  1. Reduce Feed Speed by 50%: Slow down your hand feed rate, especially as you reach the end of the cut.
  2. Support the Exit: As the blade is about to exit the stone, the thin remaining edge lacks support and will snap off, leaving a deep chip. Flip the stone or slow your feed rate to a crawl to support the exit.

3. Coolant Management

Never cut obsidian dry. Always use a generous amount of water with a rust-inhibitor additive, or light mineral oil. The coolant must flow directly onto the cutting path to flush out sharp glass shards that can scratch the blade channel.

For more scientific details and research on this topic, you can check out the International Gem Society.