Cut gemstones using cabochon and faceting techniques. Covers rough assessment, orientation for optimal color and yield, dopping, cutting angles, and crown/pavilion geometry for standard brilliant cuts. Use when you have rough gemstone material to cut into a cabochon or faceted stone, planning cutting orientation for optimal colour or optical phenomena, setting up a cabbing or faceting machine, or selecting the appropriate cutting approach for a given material.
Cut gemstones using cabochon and faceting techniques. Covers rough assessment, orientation for optimal color and yield, dopping, cutting angles, and crown/pavilion geometry for standard brilliant cuts. Use when you have rough gemstone material to cut into a cabochon or faceted stone, planning cutting orientation for optimal colour or optical phenomena, setting up a cabbing or faceting machine, or selecting the appropriate cutting approach for a given material.
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.
Confirm species identification (if uncertain, run identify-gemstone first)
Check for toxic dust risk — some materials need extra precautions
Examine for fractures under strong transmitted light
Map colour zones and inclusion locations
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)
Determine if stone is pleochroic — view through dichroscope from many directions
For phenomenal stones (star, cat's eye), locate inclusion pattern and orient accordingly
For colour-zoned material, decide whether to hide or feature zoning
Mark orientation on rough with aluminum pencil
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
Set up cabbing machine with water flow on all wheels
Put on safety glasses — no exceptions
Cut slab and mark template outline
Dop stone securely — loose stone during grinding is dangerous
Grind to shape on 80 grit, dome on 220, smooth through 600 and 1200
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)
Pick published faceting diagram for target shape and material RI
Prep rough: flat pavilion side (pointed bottom will face down)
Dop stone — use cone dop for round stones, flat dop for others
Cut pavilion facets first at published angles on coarse lap (600 mesh diamond)
Meet all pavilion facets to precise point ("culet meet")
Transfer to cone dop (or use transfer jig) to cut crown
Cut crown mains, then breaks, then stars, setting table last
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.
Clean stone thoroughly
Check symmetry: view from above (outline), from side (proportions), through table (meet precision)
For cabochons: verify dome evenness, check for flat spots, ensure consistent outline shape
For faceted stones: check meets under 10x loupe, look for facet scratches left from cutting
Measure final dimensions and weight
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
