The Challenge of Mohs 4 Minerals
Most lapidary guides are written for agate and jasper, which have a Mohs hardness of 7.0 and are highly resilient.
But what if you want to cut a vibrant purple block of Fluorite (Mohs 4) or a soft copper carbonate like Malachite (Mohs 3.5)?
Soft stones require a completely different approach. Using standard cabbing techniques will grind your stone to dust in seconds, or split it along cleavage lines.
Here is how to cab fluorite and other soft gemstones successfully.
1. Cabbing Wheels Setup: Avoid Coarse Grits
The aggressive 80-grit metal-bonded diamond wheel is the enemy of soft stones. Skip it entirely.
- Rough Shaping: Start directly on a worn 220-grit resin-bonded wheel or use a fine silicon carbide belt. Resin-bonded wheels have a cushioned backing that absorbs impact.
- Sanding Progression: Progress rapidly to 600, 1200, and 3000 grit. Keep the stone moving constantly to prevent flat spots, as material is removed very quickly.
2. Preventing Cleavage Splitting
Fluorite has perfect cleavage. If you press too hard, the crystalline structure will shear.
Cabbing Pressure Guide:
- Very Light Touch: Let the wheel do the work. Imagine you are sanding a piece of soft pine wood.
- Keep it Wet: Heat build-up from friction can cause thermal expansion, triggering a cleavage split. Ensure your water drip system is fully open and spraying directly onto the contact point.
3. Polishing Soft Minerals
Do not use cerium oxide on felt wheels for soft stones, as felt can get hot.
- Compound: Use Tin Oxide or specialized chromium oxide compounds.
- Buffer: Use a soft leather or canvas pad run at slow speed with a continuous spray of water to keep the work cold. When done correctly, fluorite will take a brilliant, glossy, glass-like shine.
For more scientific details and research on this topic, you can check out the International Gem Society.