The Art of Cutting Opal

The Art of Cutting Opal: Unlocking the Fire Within

Opal is one of the most enchanting gemstones, known for its shifting rainbow colours—what gem lovers call play-of-colour. But unlike harder stones such as sapphire or quartz, opal requires extraordinary care to cut and polish. It’s a gem that rewards patience: cut it correctly, and you’ll reveal a dazzling spectrum; cut it carelessly, and you risk losing the colour—or even the entire stone.

In this post, we’ll explore the lapidary craft of cutting opal, from raw rough to finished cabochon.

Understanding Opal Before You Cut

Every opal is unique. Unlike faceted gems, which follow predictable crystal structures, opal cutting depends on the stone’s natural colour bar and structure. Lapidarists must carefully assess:

• Colour Bar: The layers of colour within the opal, which may run parallel, perpendicular, or at odd angles to the surface.

• Potch (Common Opal): Colourless or grey opal often surrounds the colour bar. Skilled cutting removes potch to maximise visible fire.

• Cracks & Sand Inclusions: Weak points or impurities that can ruin a stone if not planned around.

• Type of Opal: Black opal, white opal, crystal opal, and boulder opal each require different approaches.

Tools of the Trade

Cutting opal requires specialised lapidary equipment:

• Trim Saw: A diamond-bladed saw to slice rough into manageable pieces.

• Dop Stick & Wax/Epoxy: Holds the stone securely while cutting and polishing.

• Diamond Grinding Wheels: Usually starting at 180–260 grit, progressing to finer wheels.

• Polishing Wheel: For the final shine, often using cerium oxide or tin oxide.

• Water Drip System: Essential to keep the stone cool and prevent cracking.

Step-by-Step: Cutting an Opal

1. Assessing the Rough

Examine the raw stone under good lighting. Look for the orientation of the colour bar—cutting too shallow or at the wrong angle can waste the gem’s fire. Sometimes the best colours are just beneath the surface.

2. Sawing

Use a diamond trim saw to cut away unwanted potch or divide the rough into sections. Always cut slowly, keeping the blade wet to reduce heat.

3. Dopping

Attach the stone to a dop stick using dop wax or a strong epoxy. This provides better control and protects your fingers during grinding.

4. Shaping (Preforming)

On a coarse diamond wheel, begin shaping the stone into a cabochon (a domed surface). The dome is critical:

• A well-rounded dome allows light to play across the colours.

• A flat stone may look dull or lifeless.

• Avoid thin edges—these are vulnerable to chipping.

5. Refining the Shape

Move to finer grit wheels (e.g., 600 grit, then 1200 grit). Smooth out scratches left by coarser grinding and refine the cabochon’s symmetry.

6. Polishing

Polish the opal on a felt or leather wheel charged with cerium oxide or tin oxide. Use gentle pressure, keeping the stone moving constantly to avoid flat spots or heat damage.

7. Inspecting the Finish

Check under strong light to ensure all scratches are gone and the polish is even. The best opals should show crisp, vibrant flashes of colour from every angle.

Special Considerations by Opal Type

• Black Opal: Usually cut as cabochons with high domes to show off colour.

• Boulder Opal: Often cut freeform, leaving natural ironstone backing for strength and contrast.

• Crystal Opal: Cut thinner, polished with care to highlight transparency and internal fire.

• White Opal: Often requires higher domes to intensify colour against the pale body tone.

Common Pitfalls in Opal Cutting

• Overheating → can cause crazing (tiny cracks).

• Overcutting → grinding past the colour bar and losing play-of-colour.

• Thin stones → vulnerable to breakage during setting.

• Inclusions left in → can weaken the gem or distract from its beauty.

Final Thoughts

Cutting opal is part science, part artistry, and part gamble. No two stones behave the same, and even experienced cutters face surprises hidden inside the rough. Yet when done well, the reward is breath taking—a gemstone alive with shifting light and colour, ready to be set in jewellery or cherished as a collector’s piece.

The true magic of opal lies in this transformation: with careful hands and patient eyes, a dull-looking rock becomes a window to a rainbow.