Cut

Cut is the characteristic of the 4 Cs that sits closest to craftsmanship. Carat, color, and clarity are mostly inherited from the rough — the cutter's influence on them is to preserve, not create. Cut is the opposite: it's what the cutter does to the stone, and it determines how well the finished diamond turns light into visible brilliance.

What the cut grade measures

GIA and most major labs grade cut on a five-step ladder: Excellent / Very Good / Good / Fair / Poor. The grade is assigned only to round brilliants on the standard report — other shapes (princess, cushion, oval, emerald, pear, marquise, radiant, heart, asscher) receive proportion assessments but no formal cut grade, because the range of acceptable proportions is wider and the measurement less standardised.

For round brilliants, the cut grade summarises three optical properties:

• Brightness — the intensity of white light returning through the crown. A bright stone looks vivid and alive; a dim one looks watery
• Fire — the dispersion of white light into spectral colours (the rainbow flashes you see when a well-cut stone moves in the light)
• Scintillation — the pattern of light and dark that moves across the stone as it tilts or the viewer shifts position

These three together produce what the trade simply calls "performance". A well-cut stone performs; a poorly cut one doesn't.

Why proportions matter

The cut grade is mostly a function of the stone's proportions — how the crown and pavilion are sized and angled relative to each other and to the overall diameter. A round brilliant is specifically engineered around a tight range of values that make the double-bounce reflection work cleanly.

Step through the three states to see the mechanism. Ideal proportions let light entering through the crown strike the pavilion at a shallow enough angle to reflect, travel across the stone, strike the opposite pavilion facet, and exit back up through the crown — a double reflection that returns most of the incoming light to the viewer.

Too shallow and the pavilion angle isn't steep enough to reflect at all. Light enters the crown, hits the pavilion below the critical angle for internal reflection, and passes straight through the bottom of the stone. The table ends up reflecting the girdle itself, producing a grey or dark ring around the centre — the classic fish-eye effect.

Too deep and light does reflect once, but the second bounce escapes through the side of the pavilion rather than upward through the crown. The centre of the table darkens — the nail-head — while the stone as a whole looks heavy and lifeless.

The sweet spot for a round brilliant is roughly:

Why cut is different from the other Cs

Cut is the one characteristic that cannot be recovered once the stone leaves the faceting wheel. Carat is fixed at rough, color and clarity are grown in by geology, but a poorly-cut stone is permanently poorly cut. Recutting is possible only at the cost of losing weight — and since per-carat price rises at weight thresholds, recutting a 1.05 ct Poor-grade stone into a 0.92 ct Excellent-grade stone often produces a lower absolute price despite the better grade.

This is why rough value depends enormously on the cutter's plan. A skilled cutter looks at a piece of rough and plans how to yield the best balance of weight retained and cut grade achieved. A poor plan can leave 15% of the rough on the table as weight savings that produce a Poor cut grade — financially worse than a smaller Excellent-grade result.

How the market prices cut

Per-carat discounts for weaker cut grades on a round brilliant, relative to Excellent of the same color and clarity:

The Very Good / Excellent gap is the most frequently-exploited arbitrage in the trade. Very Good stones are visually identical to Excellent in almost every lighting condition, priced 3–8% below, and readily available. Buyers building inventory against a "top-cut" specification rather than a "lab-certified Excellent" label consistently land better value in the VG tier.

Cut grades on non-round shapes

Non-round shapes don't receive a formal cut grade on most lab reports — the geometry is more variable and the optics more shape-specific. Instead the certificate lists proportion measurements (table, depth, length-to-width ratio for elongated shapes) and leaves the interpretation to the buyer.

For the trade, proportion assessment on non-round shapes follows shape-specific conventions:

• Princess: depth 68–75%, table 60–70%. Deeper princesses hide weight without adding brilliance
• Cushion: depth 58–66%, table 58–66%. Wide range acceptable; shape is naturally forgiving
• Oval / Pear / Marquise: length-to-width ratio drives the primary aesthetic. Ovals: 1.35–1.50. Pears: 1.45–1.75. Marquises: 1.85–2.10
• Emerald / Asscher: step-cut rather than brilliant-cut; graded on clarity transmission rather than scintillation, because large flat facets don't mask inclusions the way brilliant facets do
• Radiant: depth 65–75%, table 60–70%. Naturally bright due to brilliant faceting on a rectangular outline

For non-round shapes, the trade substitute for a cut grade is a buyer's assessment of windowing (seeing straight through the centre, indicating shallow pavilion) and extinction (dark patches where light isn't returning). Neither appears on the certificate — they're visible only in hand.

Polish and symmetry

The lab report also grades polish (how smooth and clean the facet surfaces are) and symmetry (how precisely the facets align relative to each other) on the same five-step ladder. Both affect light performance, though less dramatically than proportions:

• Polish — rough or pitted facets scatter light and dull brilliance. Poor polish is usually recoverable by re-polishing, which the cutter does if it's cost-effective
• Symmetry — misaligned facets disrupt the double-bounce pattern; the stone still returns light but the scintillation is chaotic rather than clean. Symmetry is mostly locked in at cutting and cannot be corrected

The trade sometimes refers to "Triple Excellent" — a stone graded Excellent in cut, polish, and symmetry. It's the top spec and carries a modest premium (roughly 3–7% over plain Excellent cut with lesser polish/symmetry), though the visual difference between Triple Ex and plain Excellent is marginal.

Practical sourcing notes

• Lead with cut, not certificate brand. Two stones of equal cut from different labs face up identically; two stones of the same color/clarity with different cuts do not. Cut is the one specification where buyer perception and certificate language align cleanly
• Target Very Good on cost-sensitive inventory. The 3–8% discount is routinely worth it; the visual gap is below perceptual threshold
• Avoid Good and below on commercial goods. The savings don't compensate for how much the stone loses face-up. Cut-grade shortcuts are a false economy
• For non-rounds, inspect the stone. Certificate proportions don't tell the whole story for princesses, cushions, or fancies. Windowing and extinction are only visible in hand
• Fluorescence doesn't hide poor cut. A Medium-Strong Blue stone with poor proportions still performs poorly — fluorescence interacts with color tint, not with cut geometry. Don't let one buy the other

Related

• Carat — why recutting is weight-destructive
• Color — how well-cut stones mask color tint
• Clarity — why cut partly masks inclusions face-up