Grit Progression in Stone Polishing: Why 1,200 Grit Is Where Cabochons Reveal Their Flaws
The lapidary world has a dirty secret: most cabochons fail at 1,200 grit.
The surface looks smooth enough when wet, scratches hidden beneath that thin film of water. You dry the stone, inspect under good light, and there they are—microscopic valleys that will refuse to polish no matter how much cerium oxide you throw at them. Beginners often skip grits to save time, but those skipped steps resurface three wheels later, forcing a return to square one. The progression from 80 grit to 50,000 isn't arbitrary theater. It's a cumulative process where each stage exists solely to erase the damage from the previous one.
The standard cabochon progression typically runs 80, 220, 280, 600, 1200, and 3000 grit, though that's just the foundation. Advanced work on hard stones like sapphire or garnet often extends to 14,000 and 50,000 grit diamond paste, with competition-grade pieces sometimes reaching 100,000 for that museum-quality finish. But here's what the grit numbers actually mean: lower numbers indicate larger, coarser particles that remove material quickly, while higher numbers represent finer particles that create increasingly smooth surfaces. Each stage systematically removes the scratches from the wheel before it—miss that removal, and you're polishing over damage you can't see yet.
| Grit Stage | Particle Size | Primary Function | Visual Result |
|---|---|---|---|
| 80-100 | ~180 microns | Bulk removal, initial shaping | Deep, visible scratches |
| 220-280 | ~53-63 microns | Dome refinement | Obvious scratch patterns |
| 600 | ~25 microns | Pre-polish start | Scratches visible when dry |
| 1,200 | ~15 microns | Critical inspection point | Microscopic scratches reveal themselves |
| 3,000 | ~6 microns | Fine pre-polish | Soft sheen appears |
| 8,000-14,000 | ~2-3 microns | Advanced pre-polish | Pattern emergence through haze |
| 50,000-100,000 | ~0.3 microns | Competition polish | Near-mirror finish |
The Coarse Grits: 80-280 (Shaping Territory)
For hard materials like agate, jasper, or petrified wood, most lapidaries start with 80-grit diamond or silicon carbide wheels, which rapidly remove bulk material and establish the overall cabochon outline and dome profile. The scratches at this stage are deep, obvious, and exactly what's needed for material removal.
Softer stones like turquoise may instead begin with 100 or 180 grit to avoid chipping or overcutting. The material matters. Silicon carbide at this coarse level cuts aggressively, which explains why silicon carbide wheels wear down faster than diamond wheels, though a good six-inch diamond wheel can cost three to seven times as much as an equivalent silicon carbide wheel. Silicon carbide wheels also round in the center and need regular squaring with a diamond tool to remain usable.
The 220 and 280 wheels continue refining the dome. Each wheel removes the scratches from the previous step while maintaining the cabochon's symmetry and proper curvature. This is where you establish whether the finished piece will be good or mediocre—flat spots at this stage become permanent defects.
The Mid Grits: 600-1,200 (Pre-Polish Zone)
The jump to 600 grit is where water stops being your friend during inspection. Water on the stone surface hides scratches and gives a false reading, which is why stones get dried completely before surface assessment. Good lighting makes all the difference when inspecting for scratches at this stage. By the time you finish on the 3,000 grit wheel, the surface should be very smooth with no visible scratches at all.
The quality of the pre-polish at this stage is the single most important element in getting a high polish. Here's where material differences become critical. For agate, 1,200 diamond works as a fine pre-polish, but softer gems might need 3,000 or 8,000 grit because soft materials like lapis and opal scratch readily with diamond.
If you're using silicon carbide rather than diamond, all you need is 600 grit—it wears fast and becomes the finer grit you need for a pre-polish. This is silicon carbide's party trick: it self-refines as you work. The downside is that you're running a consumable that needs constant replacement, but the upside is that it creates a natural progression within a single wheel.
The Fine Grits: 3,000-8,000 (Entry to Polish)
After the 600 or 1,200 grit stage, many lapidaries use 3,000 or 8,000 grit diamond belts or pastes in the pre-polish phase, which begin to produce a soft sheen and prepare the surface for final polishing. The stone starts looking like something at this point. Not mirror-finish, but you can see the patterns emerging through the surface haze.
The scratches at this level are microscopic. If you can still see scratches or scales after this stage, they're too deep to remove at this point, which means going back to the previous grit and starting over. This is the level where beginners discover they didn't actually finish at 1,200 like they thought they did.
When examining a cabochon, scratches or pitting reduce the amount of light reflected from the surface. The human eye is remarkably good at detecting when something's wrong with a polish, even if you can't articulate what exactly you're seeing. If the surface is properly cut, a band of reflected light glides evenly over the surface when the gem moves, but the band snakes if there are irregularities.
The Ultra-Fine Range: 14,000-50,000 (Competition Territory)
Advanced cabbers working with very hard stones like sapphire, garnet, or zircon may finish with 14,000 to 50,000 grit diamond paste, applied with a leather pad or BATT lap for superior clarity and brilliance. At this level you're not removing visible damage anymore—you're removing things that would prevent perfect polish.
The application method changes here. To use diamond paste, lapidaries squirt lubricating fluid into a small dish and add a small amount of diamond paste to form a paste, then soak the felt bob or wheel into the mixture and work it smoothly over the surface of the stone. It's a wet process that requires patience and minimal pressure. Polishing should always happen at lower RPMs, around 400-800, using minimal pressure, and polishing pads should be cleaned often to prevent dulling the final finish.
Some lapidaries go even further, using 100,000 grit diamond powder for museum-quality or competition-grade cabochons, though this step is optional for most hobbyists. At that level you're in the realm of people who enter competitions and care about winning them.
The Polishing Compounds: Cerium, Tin, and Diamond
After mechanical grinding comes the final polish, where oxides and diamond compounds take over. Cerium oxide is best for glassy or vitreous stones like quartz, obsidian, and fluorite, creating a soft chemical reaction on the surface that gently breaks down microscopic ridges while producing a brilliant polish. For more porous or soft stones like opal, turquoise, or lapis lazuli, tin oxide or aluminum oxide may be a better choice because they're gentler and less likely to undercut or stain the surface.
| Polish Type | Particle Size | Best For | Application | Working Speed |
|---|---|---|---|---|
| Cerium oxide | ~0.3 microns | Quartz, obsidian, fluorite, agate | Water paste on leather/felt | Fast, chemical + mechanical |
| Tin oxide | ~0.3 microns | Opal, turquoise, lapis, soft stones | Water paste, gentle pressure | Moderate, won't undercut |
| Aluminum oxide | ~0.3 microns | Very soft or porous materials | Water paste, minimal pressure | Slow, extremely gentle |
| Diamond paste | 0.1-3 microns | Hard stones, sapphire, garnet | Oil-based on leather/BATT | Precise, reusable |
| Linde-A | ~0.05 microns | Difficult stones, last resort | Water paste on hard felt | Very slow, ultra-fine |
Many oxide polishes are 0.3 micron or finer depending on the manufacturer, which is close to 100,000 grit. That's why they work—they're essentially the finest grit stage, but with a chemical component. Cerium is typically mixed with a little water to create a thin paste and applied generously to a leather or felt polishing surface. If cerium isn't polishing with the desired effect, tin oxide is the next choice, then Linde-A-Polish for very difficult stones.
Experienced cutters have observed that oxides often work faster than diamond, and aluminum and cerium oxides generally leave higher quality finishes. Diamond's advantage is precision and durability—diamond paste can be used repeatedly without losing its effectiveness, unlike traditional polishing pastes which wear out quickly—but oxides remain the workhorse for natural stones.
The critical rule in most workshops: only one polishing media per polishing pad, never mixing different media on the same pad. Cross-contamination ruins polish faster than anything else. Stones and hands get rinsed at every grit change during polishing, and machines get thoroughly cleaned after each grinding session to prevent contamination when moving through grit levels.
Why the Sequence Matters (And What Happens When You Skip)
The progression through finer and finer grits is necessary to remove the scratches from each previous stage—skip a grit and you'll find deep scratches that force you to go back three or four grits to remove them. It's always faster to do it right the first time.
Any grit carried over into the smoothing process will cause deep scratches in the cabochon that are impossible to polish out. This is the nightmare scenario: you're at the polish stage, the stone looks great under water, you dry it off and see a single deep scratch running across the dome. That's a rogue particle of 220 grit that hitched a ride on your fingers or didn't get washed out of the previous wheel. The stone goes back to 280, minimum. Maybe back to 220 if it's deep enough.
The systematic approach exists because stone has no forgiveness built in. Eyes become the most important feedback loop when it comes to stone polishing. When the eyes detect that something is not quite right about the surface, experienced lapidaries stop, diagnose the problem, and plan remediation steps before continuing.
The approach that works in most workshops: grind a little and look a lot. Time and observation during the polishing process separate cabochons that look machine-finished from ones that look handmade in the bad way.
Material-Specific Considerations
Most lapidary workshops and production facilities have moved away from silicon carbide to diamond grinding wheels or flat lap disks for shaping, but the progression logic remains identical. Diamond just lasts longer and maintains its grit rating more consistently. Diamond grinding discs offer superior longevity because diamonds are one of the hardest substances on Earth, while silicon carbide tends to wear down faster when used on hard or abrasive materials.
| Material Type | Starting Grit | Pre-Polish Grit | Polish Compound | Notes |
|---|---|---|---|---|
| Hard (agate, jasper, petrified wood) | 80 | 1,200 diamond | Cerium oxide | Tolerates aggressive grits |
| Medium-hard (garnet, topaz, tourmaline) | 80-100 | 3,000 diamond | Cerium or tin oxide | Standard progression |
| Soft (turquoise, lapis, opal) | 100-180 | 8,000+ or oxide only | Tin or aluminum oxide | Diamond can undercut |
| Very hard (sapphire, zircon) | 80 | 14,000-50,000 diamond | Diamond paste on leather | Competition-grade requires ultra-fine |
| Glass/vitreous (obsidian, fluorite) | 80-100 | 1,200-3,000 | Cerium oxide | Chemical polish works best |
The stone itself dictates modifications to the standard progression. Harder stones tolerate more aggressive grits earlier. Softer stones require gentler approaches throughout, sometimes starting at 180 or 220 instead of 80, and definitely using oxide polishes instead of diamond at the final stages to prevent undercutting.
What never changes: each grit exists to remove the previous grit's damage. Master that principle and the specific numbers become flexible guidelines rather than rigid rules. Fail to respect it and you'll spend more time fixing mistakes than making progress. This hands-on knowledge represents the kind of practical lapidary expertise that hobbyists now preserve as professional training programs disappear.
If you're just getting started with lapidary work, a cabbing machine with the right wheel progression will save you countless hours of frustration. And when it comes time to cut your slabs for cabbing, understanding grit progression helps you plan which blade sizes and saw blade types will work best for your projects. These aren't separate skills—they're all part of the same process of making cabochons that don't fail at inspection.