Find the useful direction
Rotate the rough under daylight-equivalent and warm light. View it through each axis, wet the surface, and use white paper or a diffused light behind it. Decide whether saturation needs to be concentrated or opened.
THE HOUSE OF CORUNDUM · CUTTING REFERENCE
CORUNDUM GEMSTONE CUTTING 101
Hard stone.
Deliberate geometry.
Faceting sapphire and ruby is the controlled removal of material to manage color, light, symmetry and weight. The machine supplies references. The cutter still makes the decisions.
- Mohs hardness
- 9
- Refractive index
- 1.762–1.770
- Approx. critical angle
- 34.4°
- Cleavage
- None
01 / KNOW THE MATERIAL
Why corundum cuts differently.
Ruby and sapphire are both corundum—crystalline aluminum oxide. Ruby is the red variety; every other gem color is sapphire. Corundum is tough, has no true cleavage and is extremely resistant to abrasion. That makes it durable in jewelry, but slower to cut and less forgiving of contamination than many beginner materials.
Hardness is not the same as toughness. A sapphire resists scratching yet can still chip at a sharp girdle, thin point, inclusion or stressed area. Heat from a dry lap, a hard knock during transfer or excessive pressure at a nearly finished meet can still cause damage.
Critical angle is a floor, not a recipe.At an RI near 1.76, the critical angle is about 34.4° from the normal. Pavilion facets near that limit can leak light because of tilt, index variation, design interactions and normal cutting error. Many corundum designs use pavilion mains around 39–42° for a practical safety margin, but the correct angles depend on the design, shape, color and desired effect.
Corundum is uniaxial and can show pleochroism—different color or intensity along different crystal directions. Orientation can make a blue sapphire look richly blue, greenish, gray or too dark. Ruby may show red versus purplish-red directions. Inspect before committing to the table plane.
02 / READ THE ROUGH
Start with the stone—not the diagram.
Map what can move
Silk may soften light beautifully; a feather near the girdle may become a durability risk. Mark surface-reaching fractures, cavities and high-stress inclusions before sawing or grinding.
Preserve valuable volume
Estimate a finished outline inside the rough in three dimensions. A slightly unusual ratio may retain far more weight than forcing a calibrated shape. Face-up beauty usually matters more than a round-number carat target.
Change the objective
Asterism and chatoyancy need a cabochon, not a brilliant. Orient the dome perpendicular to the optic axis so intersecting silk can form a centered star. Translucent material may favor a cabochon, rose cut or shallow mixed cut.
Before removing material
- Record rough weight and dimensions.
- Photograph every side.
- Mark table direction, outline and inclusions.
- Estimate depth after removing skin and fractures.
- Choose your compromise consciously: color, clarity, spread, symmetry or yield.
03 / WEIGHT RETENTION
The goal is not to save every carat.
FINISHED WEIGHT ÷ ROUGH WEIGHT × 100Weight retention %
Example: a 2.0 ct finished sapphire from 5.0 ct of rough has 40% retention.
Weight retention measures how much rough remains after cutting. It is useful for evaluating a cutting plan, but it is not a grade. A smaller, bright and durable gem can be the correct result when the alternative is a heavier stone with weak color, a window, unstable fractures or a vulnerable outline.
Challenging natural rough
Irregular shape, shallow depth, severe fractures, cavities, damaged skin or a narrow usable color zone can require major removal.
Common natural range
Normal for many pieces of faceting rough when orientation, soundness, optical performance and a balanced outline all matter.
Favorable natural rough
Possible when the crystal is clean, well-shaped, adequately deep and already close to the intended finished outline.
Purpose-grown synthetic
Clean, consistent material and regular boule shapes allow predictable placement with fewer natural defects to remove.
WHERE THE WEIGHT GOES
Every removal should solve a problem.
Color zoning and orientation
A cutter may remove pale or overly dark areas, place the richest zone beneath the table, or shorten the light path so an over-saturated sapphire does not face up black. Parti-color rough may be oriented to preserve a deliberate division of color rather than the largest possible outline.
Chips, pits and weathered skin
Surface damage must be ground away far enough to create a sound preform. A shallow chip can disappear quickly; a deep pit at the future girdle may force a smaller diameter or a different shape.
Cracks and surface-reaching fractures
Fractures are not removed merely for appearance. Those near a corner, point or girdle can open during cutting, setting or daily wear. The cutter follows the fracture to stable material—even when that costs significant weight.
Inclusions and internal stress
Natural inclusions are normal and do not all need removal. Silk, crystals and healed features may add identity or help establish natural origin. Removal becomes important when an inclusion clouds the face, interrupts light, reaches the surface or threatens durability.
Optical depth and proportions
Material is lost while establishing a pavilion deep enough to return light, a crown with useful angles and a girdle thick enough to survive setting. Retaining a shallow belly or excess pavilion weight can raise the scale reading while making the gem look smaller or windowed.
Shape correction and symmetry
Natural crystals rarely arrive centered or evenly proportioned. Bringing opposing facets into balance, correcting a tilted axis and creating a wearable outline all consume material.
NATURAL VS. SYNTHETIC
Same mineral properties. Different starting conditions.
Synthetic corundum has essentially the same chemistry, hardness and refractive behavior as natural corundum. The difference is origin: one formed in nature; the other was grown by people under controlled conditions.
Natural corundumSynthetic corundum
Irregular crystal or water-worn shapesRegular boules, rods or preforms
Color zoning and pleochroism often control orientationColor is commonly more uniform and predictable
May contain silk, healed fractures, crystals and cavitiesUsually cleaner, though growth features and strain can occur
Retention varies widely from piece to pieceRetention is usually higher and easier to plan
Each cut is an individual compromiseExcellent for learning technique and testing designs
What is normal?
For natural sapphire and ruby, losing more than half the rough weight is not automatically wasteful. A 20–40% finished yield is common, and difficult pieces may finish below 20%. Clean, blocky rough may exceed 40–50%. The meaningful question is not “How much remained?” but “Did the finished gem preserve the best color, remain durable and use the rough intelligently?”
04 / CUTTING SYSTEMS
Different references. Same geometry.
No system makes a superior gem automatically. Accuracy comes from a rigid machine, a flat lap, correct calibration and a cutter who understands how pressure, depth, angle and index interact.
Repeatability and diagram cutting
Mast machine
A faceting head travels on a fixed mast. Angle, index and height control each facet.Strengths
- Precise, repeatable settings
- Easy to return to an exact tier
- Large diagram and accessory ecosystem
- Alignment depends on mast, platen and quill geometry
- Cheater use can hide setup problems
- Higher cost and bench footprint
Fast feedback and direct control
Handheld handpiece
The indexed handpiece is lifted and placed on the lap by the cutter, with angle controlled by the handpiece mechanism.Strengths
- Immediate tactile feedback
- Fast movement between inspection and cutting
- Compact and adaptable
- Pressure and placement must stay consistent
- Requires disciplined hand control
- Repeatability depends heavily on calibration
Traditional speed and intuitive cutting
Jam-peg / jamb-peg
A dop or handpiece rests in indexed holes or notches that establish repeatable directions and approximate angles.Strengths
- Simple, durable construction
- Very fast in experienced hands
- Encourages cutting by eye and meetpoint
- Less convenient for arbitrary published diagrams
- Angle precision depends on the machine and operator
- Learning resources and parts may be regional
Preforming, carving and repair
Freehand
The stone or dop is controlled directly against a wheel or lap without a fixed index-and-angle system.Strengths
- Maximum freedom
- Excellent for shaping rough and organic forms
- Minimal equipment
- Not ideal for repeatable meetpoint faceting
- Symmetry is judged rather than mechanically indexed
- Small errors compound quickly
Which one should a beginner choose?
Mast: the easiest route into published meetpoint diagrams and repeatable tiers. Handheld: excellent for a cutter who values feel, compactness and direct control, provided the handpiece has reliable angle and index references. Jam-peg: ideal when taught by an experienced user or when traditional speed is the goal. Buy condition and alignment—not a feature list. A basic rigid machine that repeats is more useful than a digital display attached to flex or misalignment.
05 / COMPLETE WORKFLOW
From rough crystal to finished gem.
Study the rough
Clean it, weigh it, measure three axes and inspect it dry, wet, backlit and under diffused light. Mark cracks, silk, cavities, color zones and the likely table direction.
Choose the objective
Decide what matters most: color, face-up size, clean appearance, calibrated dimensions, phenomenon, yield or a specific design. You rarely maximize all of them at once.
Orient and preform
Place the strongest color and best optical direction where the finished stone needs them. Remove only material that has no realistic place in the design.
Dop and verify
Center the preform, allow the adhesive to cure fully and confirm that the dop, quill and stone run true before cutting precision tiers.
Cut the pavilion
Establish the girdle or outline, then cut pavilion tiers from coarse structure to final meets. Keep pavilion facets safely above the critical angle unless the design intentionally uses controlled leakage.
Pre-polish and polish
Remove every subsurface scratch before changing stages. Polish is not a substitute for complete pre-polish.
Transfer
Align the stone in the transfer fixture, support it without forcing, cure fully and test alignment before cutting deeply.
Cut the crown and table
Build the crown to the diagram or the stone, preserve adequate girdle thickness, then bring in and polish the table last.
Inspect and document
Check symmetry, polish, girdle, dimensions, weight and face-up performance in several lighting conditions. Record the design, angles and any deviations.
06 / ANGLES, INDEXES & DESIGNS
Optics set boundaries. Design sets the route.
Angles
Angle is normally measured from the axis perpendicular to the lap: 90° cuts the girdle, lower numbers move toward the table. Pavilion angles control reflection and depth; crown angles influence dispersion, contrast, weight and face-up color.
Indexes
The index gear divides a circle into repeatable positions. On a 120 gear, eightfold mains occur every 15 indexes: 0, 15, 30, 45, 60, 75, 90 and 105. A diagram written for another gear must be converted by proportion—not guessed facet by facet.
Meetpoints
A meetpoint is where facets are intended to intersect. Cut each facet until its meets arrive together. If one facet requires a radically different depth, investigate centering, transfer, lap flatness, mast or handpiece alignment before chasing it.
Design familyBest useTradeoff
Round brilliantReliable brightness, easy setting, calibrated productionCan sacrifice rough at corners and long axes
Oval / pearElongated rough, face-up spread, directional colorBow-tie and shoulder symmetry need control
Cushion / antique cushionBlocky rough, color retention, distinctive characterCorner and tier relationships are less forgiving
Emerald / step cutClean material, strong color zoning, long crystalsShows inclusions and polish errors clearly
Mixed cutBrilliant light return with controlled outline or colorMore tiers and more alignment opportunities
BarionDepth, scintillation and improved use of rectangular outlinesComplex meets and deeper pavilion
Rose cutShallow rough, translucency, historic styleDifferent optical goal; not a brilliant substitute
CabochonStar stones, silky, included or translucent materialNo flat-facet scintillation; dome orientation is critical
Typical working zones are starting points, not universal instructions. Many sapphire and ruby brilliant designs place pavilion facets roughly around 39–42°, crown mains around 30–36°, stars lower, and upper girdles higher. Always follow a tested design—or ray trace a new one—then adjust deliberately for the actual rough.
07 / LAPS, ABRASIVES & POLISH
Each stage must erase the one before it.
Fast preforming and major removal. Keep away from final meets and fragile tips; coarse damage can travel deeper than the visible scratch.
Establish facets and bring tiers close. On small corundum, a well-behaved 600 may be the coarsest lap needed.
Remove coarse damage and approach final meets. A fresh aggressive lap can chip; use light pressure and verify every facet.
Zinc, copper or another compatible lap can create a uniform surface that polishes quickly. Stop only when no earlier scratches remain.
Tin-alloy, Batt-type, ceramic or other proven laps may work. Corundum often responds best to diamond with light pressure, controlled lubricant and a clean lap.
What changes the result
- Lap flatness and runout
- Diamond concentration and particle size
- Oil or water carrier and the amount used
- Lap speed and sweep
- Facet direction relative to crystal orientation
- Pressure, heat and contamination
Good polish discipline
- Use separate, clearly labeled laps and applicators.
- Clean the stone, dop, hands and splash pan between stages.
- Use the least pressure that cuts predictably.
- Inspect under several light directions.
- When one facet resists, change direction, position or lap condition before increasing force.
- Never expect polish to remove a deep scratch efficiently.
08 / DOPPING & TRANSFER
Alignment errors become crown errors.
Wax is adjustable and traditional; cyanoacrylate is convenient and strong when surfaces are clean; epoxies offer working time and gap filling. Every adhesive has a release method and heat limit. Test it on expendable material before trusting an important stone.
Use a dop shape that supports the stone without contacting finished facets. During transfer, do not force a pavilion into a cone that does not match. Verify rotational alignment, centerline and tilt. After curing, cut a shallow test facet or girdle touch before committing to a deep crown tier.
If opposite crown facets meet at different depthsCheck transfer tilt, dop seating, quill alignment, lap flatness and whether the pavilion itself is centered. A cheater can correct a small rotational error; it cannot truly repair tilt or a crooked transfer.
09 / TROUBLESHOOTING
Read the symptom before changing the machine.
SymptomLikely causesFirst response
Random deep scratchesContamination, loose diamond, damaged lap or debrisStop, clean everything, inspect the lap and return to the stage that can remove the scratch.
One facet will not polishCrystal direction, subsurface damage, lap loading or poor contactChange sweep direction and lap position; refresh lightly; return to pre-polish if needed.
Meets drift around the stoneOff-center preform, inconsistent depth, flex or index errorChoose a controlling tier and recut systematically rather than correcting facets randomly.
Every facet is low on one sideMachine alignment, platen runout, lap wedge or hand pressureTest with a known-flat lap and repeatable reference before adjusting individual facets.
Facet chips at the girdleFresh coarse lap, excessive pressure, thin edge or vulnerable inclusionReduce pressure, cut toward supported material, move to a finer lap and leave a safer girdle.
Finished stone windowsPavilion too shallow or unsuitable design/RI combinationRecut pavilion at safe angles if depth permits; crown changes cannot repair a fundamentally shallow pavilion.
Stone looks blackOver-saturation, extinction, poor orientation or overly deep/closed designUse a more open design, adjust orientation, reduce path length or choose a smaller finished outline.
Table will not centerCrown tiers unequal, transfer tilt or uneven girdleBalance the surrounding crown meets first; do not enlarge the table endlessly to hide the cause.
10 / SAFETY & BENCH PRACTICE
Control water, dust, motion and chemistry.
Use adequate coolant and avoid breathing mineral or metal-lap dust. Never dry-grind corundum at the faceting bench.
Wear safety glasses when sawing, preforming, heating dops or working near a spinning lap.
Keep hair, sleeves, jewelry, cords and wipes away from the rotating platen. Stop the machine before retrieving anything.
Read adhesive, solvent and lap-alloy information. Provide ventilation, avoid skin contact and wash hands before eating.
Keep plugs and controls dry, inspect cords, use grounded protection and clean spills promptly.
Use only enough heat to release or adjust a dop. Avoid thermal shock and never assume an inclusion-bearing stone tolerates rapid heating.
11 / A PRACTICAL LEARNING PATH
Learn control before chasing complexity.
- Calibrate the machine. Confirm 90°, index repeatability, lap flatness and platen alignment.
- Start with inexpensive clean material. Synthetic corundum teaches how corundum cuts and polishes without risking valuable rough; quartz or garnet may teach meets faster but polish differently.
- Cut a tested round brilliant. Learn a consistent girdle, eightfold symmetry, meetpoints, transfer and table. Study the illustrated Standard Round Brilliant guide, then choose angles tested for the material you are cutting.
- Repeat the same design. Repetition reveals whether errors come from technique, setup or the individual stone.
- Add one variable at a time. Move to oval, cushion, step cut, included material, color zoning and then high-value natural rough.
- Keep a cutting log. Record rough size, orientation, design, indexes, angles, laps, polish, time, finished size and weight retention.
The goal is not mathematical perfection at any cost. It is a durable, beautiful gem whose geometry serves the individual piece of corundum.
Further study
Material constants and general corundum characteristics are cross-checked against the GIA sapphire reference. Cutting angles and procedures on this page are practical guidance, not a substitute for a tested design, machine instructions or hands-on training.