Cut Gemstone

---
name: cut-gemstone
locale: caveman-ultra
source_locale: en
source_commit: 82c77053
fence_basis_commit: 82c77053
translator: "Julius Brussee homage — caveman"
translation_date: "2026-04-19"
description: >
  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.
license: MIT
allowed-tools: Read
metadata:
  author: Philipp Thoss
  version: "1.0"
  domain: lapidary
  complexity: advanced
  language: natural
  tags: lapidary, cutting, faceting, cabochon, gemstones
---

# Cut Gemstone

Rough → cab / faceted stone via assessment + orientation + dopping + grinding + geometry.

## Use When

- Rough → finished cab / faceted
- Plan orientation (color, yield, phenomena)
- Setup cabbing / faceting machine
- Understand crown + pavilion angles
- Select cutting approach per material

## In

- **Required**: Rough (species ID'd — see `identify-gemstone`)
- **Required**: Approach: cab or facet
- **Required**: Target shape + size
- **Optional**: Trim saw + diamond blade
- **Optional**: Cab machine w/ 80/220/600/1200/3000 grit
- **Optional**: Faceting machine (index gear, mast, laps)
- **Optional**: Dop wax / epoxy, dop sticks, alcohol lamp
- **Optional**: Templates (oval, round, marquise) std sizes

## Do

### Step 1: Rough Assess + Safety

Evaluate rough pre-cut.

```text
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 (uncertain → `identify-gemstone` first)
2. Check toxic dust risk
3. Fractures under strong transmitted light
4. Map color zones + inclusions
5. Estimate yield shapes

**Got:** Documented assessment: species + fractures + colors + cutting plan.

**If err:** Extensive fracturing → stabilize (epoxy for porous) / yield too low → sell/trade as specimen.

### Step 2: Orientation

Optimal for color + phenomena.

```text
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. Pleochroic? → dichroscope from multi directions
2. Phenomenal (star/cat's eye) → locate inclusions, orient
3. Color-zoned → hide / feature
4. Mark orientation on rough (aluminum pencil)
5. Plan table pos + depth → max yield at orientation

**Got:** Marked rough w/ table + orientation + outline. Plan optimizes color + yield.

**If err:** Best color conflicts max yield → priority = color. Color quality > carat weight. Doubt → orient for color.

### Step 3: Cabochon Cutting

Shape → domed cab on cabbing machine.

```text
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. Setup w/ water flow all wheels
2. Safety glasses — no exceptions
3. Slab + mark template
4. Dop securely — loose stone = dangerous
5. Grind shape 80 → dome 220 → smooth 600 → 1200
6. Maintain dome curvature → uneven = "flat spots" post-polish

**Got:** Smoothly domed cab ready for polish, symmetrical, even height, no 1200-grit scratches.

**If err:** Flat spots / asymmetry → back to 220, reshape. Lose material > polish uneven. Stone off dop mid-grind → re-dop careful + check for chips.

### Step 4: Faceting

Precise geometric facets on machine.

```text
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 diagram for target shape + RI
2. Flat pavilion side (pointed bottom faces down)
3. Dop — cone for round, flat for others
4. Cut pavilion first @ angles on coarse lap (600 mesh diamond)
5. Meet all pavilion facets → precise point ("culet meet")
6. Transfer to cone dop / transfer jig → cut crown
7. Crown: mains → breaks → stars → table last
8. Pre-polish + polish each tier (see `polish-gemstone`)

**Got:** Faceted stone w/ precise meets, consistent sizes, good symmetry, proper angles for RI.

**If err:** Meets off → angles / index slightly wrong. Recheck diagram. "Chasing meets" compounds errs → re-cut tier if large. Small err = normal beginners.

### Step 5: Post-Cut Inspect

Evaluate pre-polish.

1. Clean thoroughly
2. Symmetry: above (outline) + side (proportions) + through table (meets)
3. Cabs: dome evenness + flat spots + consistent outline
4. Faceted: meets under 10x loupe + facet scratches
5. Measure final dims + weight
6. Defects → return to appropriate step pre-polish

**Got:** Fully cut stone, meets quality standards, ready for polish.

**If err:** Significant defects → re-cut now. More efficient than polish defective + re-cut. Doc what went wrong for next.

## Check

- [ ] Species ID'd + toxic dust assessed pre-cut
- [ ] Safety gear throughout (eye, dust/splash)
- [ ] Water cooling active
- [ ] Orientation planned for color / phenomena
- [ ] Cab dome symmetrical, no flat spots
- [ ] Facet meets converge to points (faceting)
- [ ] Final dims + weight recorded
- [ ] Free of cutting-stage scratches, ready for polish

## Traps

- **Cut unidentified stone**: Toxic dust (malachite, cinnabar, chrysotile). Always ID before. Water cooling always.
- **Skip orientation**: Pleochroic cut w/o orientation → dull/off-color vs beautiful if correct.
- **Dry grind**: Silica dust (chronic hazard) + overheats (thermal shock fractures). Water flow continuously.
- **Rush grits**: 220 → 1200 skips → deep scratches show post-polish. Each grit fully removes prior scratches.
- **Dopping fail**: Inadequate adhesion → shift / fly off. Enough wax, warm stone, cool dop fully pre-grind.

## →

- `identify-gemstone` — ID required pre-cut
- `polish-gemstone` — next step, lap + compound + finish