Cut Gemstone

Cut gemstones from rough material using cabochon and faceting techniques, including rough assessment, orientation planning, dopping, grinding, and faceting geometry.

When to Use

• You have rough gemstone material and want to cut it into a finished cabochon or faceted stone
• You need to plan cutting orientation for optimal colour, yield, or optical phenomena
• You are setting up a cabbing machine or faceting machine for the first time
• You want to understand crown and pavilion angles for standard brilliant cuts
• You need to select the appropriate cutting approach for a given material

Inputs

• Required: Rough gemstone material (identified species — see identify-gemstone)
• Required: Cutting approach decision: cabochon or faceting
• Required: Target shape and approximate size
• Optional: Trim saw with diamond blade
• Optional: Cabbing machine with 80/220/600/1200/3000 grit wheels (for cabochons)
• Optional: Faceting machine with index gear, mast, and lap set (for faceting)
• Optional: Dop wax or epoxy, dop sticks, alcohol lamp
• Optional: Templates (oval, round, marquise) in standard calibrated sizes

Procedure

Step 1: Rough Assessment and Safety Check

Evaluate the rough material before any cutting begins.

Rough Assessment Checklist:
+--------------------+------------------------------------------+
| Factor             | Assessment                               |
+--------------------+------------------------------------------+
| Species            | Identified? (MANDATORY before cutting)   |
|                    | Toxic dust risk? (check below)           |
+--------------------+------------------------------------------+
| Fractures          | Internal fractures that limit yield?     |
|                    | Will the stone break during cutting?     |
+--------------------+------------------------------------------+
| Colour zones       | Where is the best colour concentrated?   |
|                    | Can the cut centre the colour?           |
+--------------------+------------------------------------------+
| Inclusions         | Large inclusions that should be cut away?|
|                    | Silk for star stones? (orient for star)  |
+--------------------+------------------------------------------+
| Size and shape     | What finished shapes fit this rough?     |
|                    | Calibrated size possible?                |
+--------------------+------------------------------------------+
| Yield estimate     | Approximate finished weight as % of rough|
|                    | Typical: 25-40% for faceting             |
|                    | Typical: 40-60% for cabochons            |
+--------------------+------------------------------------------+

SAFETY — TOXIC DUST MATERIALS:
These minerals produce hazardous dust when cut. Use wet cutting ONLY,
ensure ventilation, and wear an appropriate respirator:
- Chrysotile (asbestos serpentine) — NEVER cut dry
- Malachite — copper carbonate dust is toxic
- Cinnabar — mercury sulfide, extremely toxic
- Orpiment/Realgar — arsenic compounds
- Chrysocolla — copper silicate, moderate risk
- Tiger's eye (fibrous) — fine silica fibers

ALL stone cutting produces silica dust. Always use water cooling
and never grind or cut dry without a dust extraction system.

1. Confirm the species identification (if uncertain, run identify-gemstone first)
2. Check for toxic dust risk — some materials require extra precautions
3. Examine for fractures under strong transmitted light
4. Map colour zones and inclusion locations
5. Estimate what finished shapes the rough can yield

Expected: A documented rough assessment with species confirmed, fractures mapped, colour zones identified, and a cutting plan formed.

On failure: If the rough has extensive fracturing, consider whether it can be stabilized (epoxy impregnation for porous material) or if the yield is too low to justify cutting. Some rough is better sold or traded as specimen material.

Step 2: Orientation Planning

Determine the optimal cutting orientation for colour and phenomena.

Orientation Principles by Stone Type:

PLEOCHROIC STONES (tourmaline, sapphire, tanzanite, iolite):
- Orient the table perpendicular to the crystal axis showing
  the best face-up colour
- Tourmaline: the c-axis often shows dark/opaque colour —
  orient the table to view the a/b axis colour
- Sapphire: slight pleochroism — orient for deepest blue face-up
- Tanzanite: trichroic — blue/violet axis preferred for table

STAR STONES (star ruby, star sapphire):
- Silk (rutile needles) must be parallel to the base
- Cut as cabochon with the dome centred over the silk
- The star appears at 90 degrees to the silk orientation

CAT'S EYE STONES (chrysoberyl cat's eye, tiger's eye):
- Fibrous inclusions must run perpendicular to the length
  of an elongated cabochon
- The eye appears as a bright line across the shortest dimension

COLOUR-ZONED MATERIAL (sapphire, ametrine, watermelon tourmaline):
- Position colour zones so they are not visible face-up
- Or feature them intentionally (ametrine, watermelon tourmaline)

1. Determine if the stone is pleochroic — view through the dichroscope from multiple directions
2. For phenomenal stones (star, cat's eye), locate the inclusion pattern and orient accordingly
3. For colour-zoned material, decide whether to hide or feature the zoning
4. Mark the orientation on the rough with an aluminum pencil
5. Plan the table position and depth to maximise yield at the chosen orientation

Expected: A marked rough stone with the table direction, orientation, and approximate outline indicated. The cutting plan optimises colour presentation and yield.

On failure: If the best colour orientation conflicts with maximum yield, decide based on priority: colour quality almost always increases value more than additional carat weight. When in doubt, orient for colour.

Step 3: Cabochon Cutting

Shape a gemstone into a domed cabochon on a cabbing machine.

Cabochon Cutting Sequence:

EQUIPMENT SETUP:
- Cabbing machine with water drip on all wheels
- Wheel sequence: 80, 220, 600, 1200, 3000 (or 1200 + polish)
- Dop sticks and dop wax (or cyanoacrylate adhesive)
- Safety glasses — MANDATORY
- Avoid loose clothing, tie back long hair

STEP-BY-STEP:
1. SLAB: Cut a slab 5-8mm thick through the best area
2. TEMPLATE: Mark the desired outline (oval, round, etc.)
   using a template and aluminum pencil
3. TRIM: Remove excess material on the trim saw or 80-grit wheel
   Cut close to the line but leave 1-2mm margin
4. DOP: Attach the slab to a dop stick with dop wax
   Heat the wax, press the stone flat-side down, centre it
5. SHAPE (80 grit): Grind to the template outline
   Work all the way around, maintaining symmetry
6. DOME (220 grit): Shape the dome profile
   Standard dome height = ~1/3 of the stone's width
   Keep the dome symmetrical — check from all angles
7. SMOOTH (600 grit): Remove 220-grit scratches
   Work systematically, keeping even pressure
8. PRE-POLISH (1200 grit): Remove 600-grit scratches
   The surface should feel smooth to the fingernail
9. FLAT BOTTOM: Remove the stone from the dop, re-dop
   face-down, and grind the bottom flat on 220 → 600 grit
10. POLISH: See polish-gemstone skill for final finishing

1. Set up the cabbing machine with water flow on all wheels
2. Put on safety glasses — no exceptions
3. Cut a slab and mark the template outline
4. Dop the stone securely — a loose stone during grinding is dangerous
5. Grind to shape on 80 grit, dome on 220, smooth through 600 and 1200
6. Maintain consistent dome curvature throughout — uneven domes show "flat spots" after polishing

Expected: A smoothly domed cabochon ready for final polishing, with symmetrical outline, even dome height, and no visible scratches from the 1200-grit stage.

On failure: If the dome has flat spots or asymmetry, return to 220 grit and reshape. Better to lose a little material than to polish an uneven dome. If the stone comes off the dop during grinding, re-dop carefully and continue — check the stone for chips first.

Step 4: Faceting

Cut precise geometric facets using a faceting machine.

Standard Round Brilliant Angles (quartz-family, RI ~1.54):
+------------------+-------+--------+
| Facet            | Angle | Index  |
+------------------+-------+--------+
| Crown main       | 42°   | 96-index: 3,9,15,21,27,33,39,45 |
| Crown break      | 25°   | (bisect mains)                   |
| Crown star       | 15°   | (bisect breaks toward table)     |
| Table            | 0°    | flat    |
| Pavilion main    | 43°   | 96-index: 3,9,15,21,27,33,39,45 |
| Pavilion break   | Use GemCad or published diagrams           |
+------------------+-------+--------+

Standard Round Brilliant Angles (corundum, RI ~1.76):
+------------------+-------+
| Facet            | Angle |
+------------------+-------+
| Crown main       | 37°   |
| Pavilion main    | 41°   |
+------------------+-------+

CRITICAL: Pavilion angles determine brilliance.
- Too shallow → light leaks through bottom ("windowing")
- Too steep → dark extinction zones
- Correct angle → total internal reflection (brilliance)

1. Select a published faceting diagram for the target shape and material RI
2. Prepare the rough: flat the pavilion side (the pointed bottom will face down)
3. Dop the stone — use a cone dop for round stones, flat dop for others
4. Cut pavilion facets first at the published angles on the coarse lap (600 mesh diamond)
5. Meet all pavilion facets to a precise point (the "culet meet")
6. Transfer to a cone dop (or use a transfer jig) to cut the crown
7. Cut crown mains, then breaks, then stars, establishing the table last
8. Pre-polish and polish each tier (see polish-gemstone for lap and compound selection)

Expected: A faceted gemstone with precise meets (where facet edges converge to a single point), consistent facet sizes, good symmetry, and proper angles for the material's RI.

On failure: If facet meets are off, the angles or index settings are slightly wrong. Re-check the published diagram. "Chasing meets" (adjusting one facet to fix another) compounds errors — it is better to re-cut the tier if the error is large. Small meet errors are normal for beginners and do not significantly affect brilliance.

Step 5: Post-Cut Inspection

Evaluate the cut stone before proceeding to final polish.

1. Clean the stone thoroughly
2. Check symmetry: view from above (outline), from the side (proportions), and through the table (meet precision)
3. For cabochons: verify dome evenness, check for flat spots, ensure consistent outline shape
4. For faceted stones: check meets under 10x loupe, look for facet scratches remaining from cutting
5. Measure final dimensions and weight
6. If defects are found, return to the appropriate cutting step before polishing

Expected: A fully cut stone that meets quality standards for symmetry, meets, and surface preparation, ready for the polishing stage.

On failure: If significant defects are found (poor symmetry, bad meets, incorrect proportions), it is more time-efficient to re-cut now than to polish a defective stone and re-cut later. Document what went wrong for the next stone.

Validation

• Species identified and toxic dust risk assessed before cutting
• Safety equipment worn throughout (eye protection, dust/splash control)
• Water cooling active on all grinding and cutting operations
• Orientation planned for optimal colour or phenomena
• Cabochon dome is symmetrical with no flat spots (cabochon path)
• Facet meets converge to points without significant offset (faceting path)
• Final dimensions measured and recorded
• Stone is free of cutting-stage scratches and ready for polishing

Common Pitfalls

• Cutting an unidentified stone: Some materials produce toxic dust (malachite, cinnabar, chrysotile). Always identify before cutting. Always use water cooling regardless
• Skipping orientation planning: Cutting a pleochroic stone without orienting for colour can produce a dull or off-colour finished gem that would have been beautiful with correct orientation
• Grinding dry: Dry grinding produces silica dust (chronic health hazard) and overheats the stone (thermal shock can fracture it). Water must flow continuously on all wheels and laps
• Rushing through grits: Skipping from 220 to 1200 grit leaves deep scratches that show after polishing. Each grit stage must fully remove the scratches from the previous stage
• Dopping failure: Inadequate dop adhesion causes the stone to shift or fly off during grinding. Use enough wax, ensure the stone is warm (not cold), and let the dop cool completely before grinding

Related Skills

• identify-gemstone — Species identification is required before cutting begins
• polish-gemstone — The next step after cutting, covering lap selection, compound choice, and final finish