Cut Gemstone

Cut gemstones from rough material. Cabochon and faceting techniques. Rough assessment, orientation planning, dopping, grinding, faceting geometry.

When Use

• Have rough gemstone material. Want to cut into finished cabochon or faceted stone
• Need to plan cutting orientation for optimal colour, yield, optical phenomena
• Setting up cabbing machine or faceting machine for first time
• Want to understand crown and pavilion angles for standard brilliant cuts
• Need to pick right cutting approach for 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

Steps

Step 1: Rough Assessment and Safety Check

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

Got: Documented rough assessment with species confirmed, fractures mapped, colour zones identified, cutting plan formed.

If fail: Rough has extensive fracturing? Consider whether it can be stabilized (epoxy impregnation for porous material) or if yield too low to justify cutting. Some rough better sold or traded as specimen material.

Step 2: Orientation Planning

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

Got: Marked rough stone with table direction, orientation, approximate outline indicated. Cutting plan optimises colour presentation and yield.

If fail: Best colour orientation conflicts with max yield? Decide based on priority: colour quality almost always grows value more than extra carat weight. In doubt, orient for colour.

Step 3: Cabochon Cutting

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

Got: Smoothly domed cabochon ready for final polishing. Symmetrical outline, even dome height, no visible scratches from 1200-grit stage.

If fail: Dome has flat spots or asymmetry? Return to 220 grit and reshape. Better to lose little material than polish uneven dome. Stone comes off dop during grinding? Re-dop carefully and continue — check stone for chips first.

Step 4: Faceting

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

Got: Faceted gemstone with precise meets (where facet edges converge to single point), consistent facet sizes, good symmetry, proper angles for material's RI.

If fail: Facet meets off? Angles or index settings slightly wrong. Re-check published diagram. "Chasing meets" (adjusting one facet to fix another) compounds errors — better to re-cut tier if error is large. Small meet errors normal for beginners and do not big affect brilliance.

Step 5: Post-Cut Inspection

Evaluate cut stone before going to final polish.

1. Clean stone thoroughly
2. Check symmetry: view from above (outline), from side (proportions), through 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 left from cutting
5. Measure final dimensions and weight
6. Defects found? Return to right cutting step before polishing

Got: Fully cut stone meeting quality standards for symmetry, meets, surface preparation. Ready for polishing stage.

If fail: Big defects found (poor symmetry, bad meets, wrong proportions)? More time-efficient to re-cut now than polish defective stone and re-cut later. Document what went wrong for next stone.

Checks

• 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 symmetrical with no flat spots (cabochon path)
• Facet meets converge to points without big offset (faceting path)
• Final dimensions measured and recorded
• Stone free of cutting-stage scratches. Ready for polishing

Pitfalls

• Cutting an unidentified stone: Some materials produce toxic dust (malachite, cinnabar, chrysotile). Always identify before cutting. Always use water cooling anyway
• Skipping orientation planning: Cutting pleochroic stone without orienting for colour → dull or off-colour finished gem that would have been beautiful with right orientation
• Grinding dry: Dry grinding produces silica dust (chronic health hazard) and overheats 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 showing after polishing. Each grit stage must fully remove scratches from previous stage
• Dopping failure: Weak dop adhesion → stone shifts or flies off during grinding. Use enough wax, keep stone warm (not cold), let dop cool completely before grinding

See Also

• identify-gemstone — Species identification required before cutting begins
• polish-gemstone — Next step after cutting. Covers lap selection, compound choice, final finish