polish-gemstone

Polish Gemstone

Polish cut gemstones to a final optical-quality finish using progressive abrasive sequences, appropriate polishing compounds, and correct lap or pad selection for both cabochons and faceted stones.

When to Use

• You have a cabochon or faceted stone that has completed the cutting stage and is ready for polishing
• You need to select the correct polishing compound and lap for a specific gemstone species
• You are troubleshooting a polish that will not reach full lustre (orange peel, haze, scratches)
• You want to re-polish an existing stone that has surface wear or scratches

Inputs

• Required: A cut gemstone ready for polishing (cabochon dome shaped and smoothed, or faceted stone with all facets cut)
• Required: Species identification (polishing compound and lap selection depend on the material)
• Optional: Cabbing machine with polishing wheels/pads (for cabochons)
• Optional: Faceting machine with polishing laps (for faceted stones)
• Optional: Polishing compounds: cerium oxide, aluminum oxide, diamond paste (various micron grades), tin oxide, chromium oxide
• Optional: Polishing laps: tin, copper, ceramic, Batt (synthetic), BATT lap, Corian, Lucite, leather, felt
• Optional: 10x loupe or gemological microscope for quality inspection

Procedure

Step 1: Surface Preparation

Ensure the stone's surface is properly prepared from the cutting stage.

Pre-Polish Surface Check:
+--------------------+------------------------------------------+
| Check              | Requirement                              |
+--------------------+------------------------------------------+
| Previous grit      | All scratches from the cutting stage     |
| scratches          | must be removed by the final grit        |
|                    | (typically 1200 or 3000)                 |
+--------------------+------------------------------------------+
| Surface uniformity | No flat spots on cabochons, no uneven    |
|                    | facets on faceted stones                 |
+--------------------+------------------------------------------+
| Cleanliness        | Stone thoroughly cleaned between grits   |
|                    | and before polishing. Grit contamination |
|                    | is the #1 cause of polish failure        |
+--------------------+------------------------------------------+
| Dop security       | Stone securely dopped — shifting during  |
|                    | polish destroys facet geometry           |
+--------------------+------------------------------------------+

1. Examine the stone under 10x magnification
2. Look for remaining scratches from the cutting grits — they appear as parallel lines
3. If scratches remain, return to the appropriate grit stage before polishing
4. Clean the stone thoroughly: ultrasonic cleaner, or brush with dish soap and water
5. Clean your hands, the dop, and the work area — grit contamination transfers easily

Expected: A scratch-free surface at the pre-polish grit level, thoroughly cleaned, with secure dopping. The surface should feel glass-smooth to the fingernail.

On failure: If scratches persist after repeated grinding at the fine grit, the stone may have directional hardness (common in corundum and kyanite). Try changing the grinding direction. If contamination is the issue, clean everything and replace the water supply.

Step 2: Pre-Polish Sequence (Cabochons)

For cabochon stones, work through the pre-polish grit stages.

Cabochon Pre-Polish Sequence:
+------+-----------+------------------------------------------+
| Stage| Grit      | Purpose                                  |
+------+-----------+------------------------------------------+
| 1    | 220       | Shape and dome (cutting stage — done)    |
+------+-----------+------------------------------------------+
| 2    | 600       | Remove 220 scratches, refine shape       |
+------+-----------+------------------------------------------+
| 3    | 1200      | Remove 600 scratches, smooth surface     |
+------+-----------+------------------------------------------+
| 4    | 3000      | Remove 1200 scratches (optional but      |
|      | (or 1500) | recommended for hard stones like agate)  |
+------+-----------+------------------------------------------+
| 5    | 8000      | Pre-polish (some machines include this)   |
|      | (or 14000)| Fine diamond paste on appropriate pad     |
+------+-----------+------------------------------------------+

TIME PER GRIT: Spend equal time at each stage. Rushing a grit
stage means scratches carry forward and become visible after
polishing. A typical cabochon takes 3-5 minutes per grit stage.

1. Work through each grit stage in order — never skip a grit
2. At each stage, examine the stone under magnification to confirm all previous-grit scratches are removed before advancing
3. Clean the stone and your hands thoroughly between grits
4. Maintain consistent pressure and motion across the entire surface
5. Keep water flowing to prevent heat buildup

Expected: After completing the pre-polish sequence, the stone should have a uniform satin-smooth surface with no visible scratches under 10x magnification.

On failure: If scratches persist at a given grit, continue working at that grit — do not advance. If deep scratches will not come out, drop back one grit stage and re-grind. Contamination from coarser grits is the most common cause of persistent scratching.

Step 3: Polishing Compound and Lap Selection

Select the correct polishing compound and lap for the gemstone species.

Polishing Compound Guide:
+-------------------+------------------------------------------+
| Compound          | Best For                                 |
+-------------------+------------------------------------------+
| Cerium oxide      | Quartz family (agate, jasper, amethyst,  |
|                   | chalcedony), feldspar, obsidian          |
|                   | Mix: paste consistency with water        |
|                   | Lap/pad: leather, felt, or Batt          |
+-------------------+------------------------------------------+
| Diamond paste     | Universal — works on everything          |
| (50,000 mesh /    | Essential for corundum, spinel, topaz,   |
| 0.25-0.5 micron)  | garnet, and other hard stones            |
|                   | Lap: tin, copper, ceramic, Corian        |
+-------------------+------------------------------------------+
| Aluminum oxide    | Corundum (ruby, sapphire), spinel,       |
| (0.3 micron)      | chrysoberyl                              |
|                   | Lap: ceramic, tin, wax                   |
+-------------------+------------------------------------------+
| Tin oxide          | Quartz varieties, opal (gentle polish)  |
|                   | Lap: leather, felt, Lucite              |
+-------------------+------------------------------------------+
| Chromium oxide    | Jade (jadeite, nephrite), emerald        |
| (green compound)  | Lap: leather                             |
+-------------------+------------------------------------------+
| Linde A           | Corundum, spinel — traditional choice    |
| (aluminum oxide)  | Lap: wax, ceramic, tin                   |
+-------------------+------------------------------------------+

Polishing Lap Guide (Faceted Stones):
+-------------------+------------------------------------------+
| Lap Material      | Use                                      |
+-------------------+------------------------------------------+
| Tin (type metal)  | General-purpose polish lap. Good for     |
|                   | most stones with diamond or alumina      |
+-------------------+------------------------------------------+
| Copper             | Diamond polish for hard stones          |
|                   | (corundum, spinel, topaz)                |
+-------------------+------------------------------------------+
| Ceramic (BATT,    | Diamond polish. Forgiving, good for     |
| Darkside, Last    | beginners. Works on most materials       |
| Lap)              |                                          |
+-------------------+------------------------------------------+
| Corian (solid     | Oxide polishes. Good for quartz family   |
| surface)          |                                          |
+-------------------+------------------------------------------+
| Lucite/Plexiglass | Oxide polish for quartz, softer stones   |
+-------------------+------------------------------------------+
| Leather/felt      | Cabochon polishing. Cerium oxide or      |
|                   | tin oxide                                |
+-------------------+------------------------------------------+

1. Match the polishing compound to the gemstone species
2. Select the appropriate lap or pad material
3. For unknown or uncommon stones, diamond paste on a ceramic lap is the safest starting choice
4. Prepare the compound: mix oxides to a thin paste with water; apply diamond paste sparingly to the lap
5. Apply compound evenly to the lap — excess compound causes "orange peel" texture

Expected: A compound + lap combination appropriate for the gemstone species. Using the wrong combination wastes time and can produce a sub-optimal finish.

On failure: If the standard compound does not produce a good polish on the first attempt, try: (1) a different lap material, (2) a finer compound grade, or (3) reducing pressure. Some stones are sensitive to polishing speed — try slower rpm. If all else fails, diamond paste on tin or ceramic works on virtually any material.

Step 4: Final Polishing

Execute the polishing process.

Polishing Technique:

CABOCHON POLISHING:
1. Apply compound to the leather/felt wheel
2. Run the wheel at moderate speed (lower than grinding)
3. Hold the stone lightly — REDUCE pressure compared to grinding
4. Move the stone across the wheel surface with gentle sweeping motion
5. Polish for 2-5 minutes, checking progress with a loupe
6. Add compound sparingly — too much creates orange peel

FACETED STONE POLISHING:
1. Charge the polishing lap with compound
2. Set the SAME angle as the cutting stage for each facet tier
3. Lower the stone gently onto the spinning lap
4. Polish each facet with light, consistent pressure
5. Check each facet under a loupe before moving to the next
6. Re-charge the lap periodically but do not over-charge

SPEED AND PRESSURE:
- Polishing speed: 50-75% of cutting speed
- Pressure: LIGHT — let the compound do the work
- Heavy pressure causes heat, orange peel, and facet rounding
- On faceted stones, heavy pressure rounds facet edges ("soft meets")

1. Apply compound to the lap/pad
2. Set the correct angle (faceted stones) or hold at the dome angle (cabochons)
3. Polish with light, even pressure
4. Check progress every 1-2 minutes under 10x magnification
5. Continue until the surface shows full lustre with no remaining scratches
6. Final rinse: clean the stone thoroughly to remove all compound residue

Expected: A fully polished surface with mirror-like lustre (faceted stones) or deep, even lustre (cabochons). No scratches visible under 10x magnification. Facet edges are sharp and meets are crisp.

On failure: Common polish problems and solutions:

• Orange peel (textured surface): Too much compound, too much pressure, or contaminated lap. Clean the lap and re-apply compound sparingly
• Persistent scratches: Contamination from coarser grit. Clean everything, check compound for contamination, re-polish
• Soft meets (rounded facet edges): Too much pressure. Reduce pressure and use a harder lap
• Haze (no full lustre): Wrong compound for the material, or compound too dry. Try a different compound or add water/extender

Step 5: Quality Assessment

Inspect the finished stone under magnification.

Final Quality Checklist:
+--------------------+------------------------------------------+
| Criterion          | Standard                                 |
+--------------------+------------------------------------------+
| Lustre             | Full, even lustre across all surfaces    |
|                    | No dull patches or haze                  |
+--------------------+------------------------------------------+
| Scratches          | None visible under 10x magnification     |
|                    | Check under multiple light angles        |
+--------------------+------------------------------------------+
| Orange peel        | None — surface should be optically flat  |
|                    | on each facet / smoothly curved on cabs  |
+--------------------+------------------------------------------+
| Facet edges        | Sharp and well-defined (faceted stones)  |
| (faceted)          | No rounding or "soft meets"              |
+--------------------+------------------------------------------+
| Dome uniformity    | Smooth, even curvature (cabochons)       |
| (cabochons)        | No flat spots or high points             |
+--------------------+------------------------------------------+
| Cleanliness        | All compound residue removed             |
|                    | Stone cleaned ultrasonically or by hand  |
+--------------------+------------------------------------------+

1. Clean the stone thoroughly — remaining compound can mimic a polish
2. Examine under a bright, directional light source at multiple angles
3. Use 10x magnification to check for remaining scratches
4. For faceted stones: check each facet individually and verify meet precision
5. For cabochons: verify the dome reflects a single, undistorted light point (the "light return test")
6. Record final weight and dimensions

Expected: A gemstone with professional-quality polish — full lustre, no visible scratches under 10x, sharp facet edges or smooth dome, and clean of all residue. Ready for setting, display, or sale.

On failure: If specific areas fail inspection, you can spot-polish individual facets (faceted stones) or return to the polishing wheel for specific areas (cabochons). Do not re-polish the entire stone unless the defect is widespread.

Validation

• Pre-polish surface was scratch-free before polishing began
• Stone and equipment cleaned between every grit stage
• Correct polishing compound selected for the gemstone species
• Appropriate lap or pad material used
• Light pressure maintained throughout polishing
• No scratches visible under 10x magnification in final inspection
• Full lustre achieved across all surfaces
• Facet edges sharp (faceted) or dome smooth (cabochon)
• All compound residue removed in final cleaning

Common Pitfalls

• Grit contamination: The single most common cause of polish failure. A single grain of 220 grit on a polishing lap creates deep scratches. Clean everything obsessively between stages
• Too much pressure: Heavy pressure generates heat (can crack the stone), causes orange peel texture, and rounds facet edges. Let the compound do the work — pressure should be barely more than the stone's own weight
• Too much compound: Over-charging the lap creates a slurry layer that produces orange peel rather than a flat polish. Apply compound sparingly and re-charge periodically rather than loading the lap
• Wrong compound for the species: Cerium oxide works beautifully on quartz but poorly on corundum. Diamond paste works on everything but is expensive. Match the compound to the stone
• Skipping the inspection: Always check under 10x before declaring the polish complete. Defects invisible to the naked eye become obvious once the stone is set in jewellery or examined by a buyer

Related Skills

• cut-gemstone — The cutting stage must be completed properly before polishing begins; scratches carried from cutting cannot be fixed during polishing alone
• appraise-gemstone — Polish quality directly affects the "cut" grade in gemstone appraisal, particularly brilliance and surface finish assessment