What Is an Ideal Cut Diamond? Table & Depth Explained

• IGI-certified diamonds
• Triple Excellent standard
• Lifetime setting upgrades
• Free, fully insured shipping
• Made to order in Toronto

An ideal cut diamond is one whose proportions — chiefly its table and depth percentages — are tuned so that light enters the stone, reflects internally, and returns to your eye as brilliance. Get the proportions right and the diamond is alive; get them wrong and it looks flat, whatever its size, colour or clarity. Here is what table and depth actually mean, the physics that makes the ideal window so narrow, and the ranges Montare holds for every shape.

Key takeaways

• Table % is the width of the flat top facet relative to the stone's width.
• Depth % is how deep the stone is relative to its width.
• Both must sit in a narrow ideal range for maximum light return.
• Outside that range light leaks, and the diamond looks dull, dark or glassy.
• Ideal ranges differ by shape — there is no single correct number.
• An Excellent cut grade is broader than the ideal window, so the two are not the same thing.

What is the table of a diamond?

The table is the large flat facet on the very top of the stone — the window that light enters and, if the diamond is cut well, returns through. Table percentage expresses that facet's width as a share of the stone's total width. A round brilliant measuring 9.4 mm across with a 5.2 mm table has a table percentage of roughly 55%.

The size of that window is a trade-off between two kinds of beauty. A larger table admits more light and pushes white brilliance up, but it does so by shrinking the crown facets that split light into spectral colours, so the stone shows less fire and can read slightly glassy. A smaller table preserves fire but reduces the aperture, and the stone loses brightness. The ideal band is where both survive.

What is depth percentage?

Depth is the height of the stone from the table down to the culet at the bottom, expressed as a percentage of its width. It is the measurement that decides where the light goes once it is inside.

Depth also governs something buyers feel more directly: face-up size. Carat is weight, not dimensions, and a diamond cut too deep carries much of that weight hidden below the girdle where nobody will ever see it. Two stones of identical carat weight can differ visibly in width across the finger purely because one was cut deep to preserve weight from the rough and the other was cut to spread. This is the mechanism behind the point we make in carat versus size: what you pay for is weight, but what you see is spread.

Why the ideal window is so narrow

The narrowness is not a matter of convention. It follows from the optics of the material.

Diamond has a refractive index of about 2.42 — extraordinarily high — which gives it a critical angle of roughly 24.4 degrees. Light hitting an internal facet at an angle shallower than that does not reflect; it refracts straight out of the stone and is lost. Everything a cutter does with table and depth is an attempt to keep light striking the pavilion facets above that threshold, twice, so it exits back through the table.

Two failure modes sit on either side. Too shallow, and light passes out through the bottom rather than reflecting — the stone develops a pale, transparent patch under the table that cutters call a window, and it looks watery. Too deep, and light does reflect but is directed toward the sides rather than up, so it exits where nobody is looking; the stone reads dark at the middle, and it wears smaller than its weight. For a round brilliant, optical modelling puts the ideal pavilion main angle at roughly 40.75 to 41 degrees, and the table and depth percentages below are what deliver it in practice.

Ideal table and depth by shape

There is no single ideal, because the facet architecture differs by shape. Step cuts such as the emerald use broad parallel facets and need a wider table; brilliant cuts need a narrower one. These are the ranges every Montare diamond must sit inside, alongside the length-to-width ratio and the approximate face-up spread at our three-carat floor.

Shape	Table %	Depth %	Ideal L:W	3ct spread (approx.)
Round	54–57	59–62.5	1.00	9.4 mm
Oval	54–58	58–62	1.35–1.50	11.0 × 8.0 mm
Cushion	60–62	61–67	1.25–1.35	10.0 × 7.7 mm
Emerald	62–66	61–67	1.30–1.50	9.4 × 6.6 mm
Radiant	62–66	61–67	1.20–1.35	9.1 × 7.2 mm
Pear	54–58	58–62	1.45–1.70	12.5 × 8.0 mm
Marquise	54–58	58–62	1.85–2.10	14.0 × 7.0 mm

Two notes on that table. Montare's cushion and radiant are elongated only — we do not offer square versions of either, which is why both ratio bands start well above 1.00. And table and depth percentages are normalised to the stone's width, which means they do not shift as the length-to-width ratio changes; a longer oval and a rounder oval share the same ideal depth band.

How does this relate to the cut grade?

Table and depth are the heart of the cut grade, but they are not the whole of it, and — importantly — the grade is more forgiving than the ideal window. An Excellent cut grade covers a broader span of proportions than "ideal" does, so two stones can both be graded Excellent while one sits in the middle of the ideal band and the other sits at its edge.

That gap is the reason Montare states two conditions rather than one. Every stone must be Triple Excellent — Excellent in cut, polish and symmetry — and sit inside the ideal table and depth range for its own shape. The grade confirms craftsmanship; the proportions confirm the optics. For a closer look at either number on its own, see our guides to table percentage by shape and depth percentage by shape.

How to check proportions on a certificate

Both figures are printed on every IGI report, usually in the proportions diagram beside the plotted stone, expressed as percentages rather than millimetres. Three quick checks are worth making before anything else.

Compare against the shape, not a universal number. A 65% table is excellent on an emerald cut and poor on a round. Most disappointment comes from applying round-brilliant figures to fancy shapes.

Read the measurements line as well. It gives the actual millimetre dimensions, which is how you catch a stone carrying hidden weight: if the spread is well below the figures above for its carat weight, the stone was cut deep.

Check all three cut-related grades, not one. Cut, polish and symmetry are printed separately, and a stone sold as "Excellent cut" may be a grade lower on the other two. Our walkthrough of how to read a diamond certificate covers the whole report.

The Montare standard

Every Montare diamond is cut to ideal table and depth for its shape, Triple Excellent, D–E colour, VVS1–VVS2 clarity, three carats minimum and IGI certified. The proportions that decide brilliance are the easiest part of a diamond to compromise quietly, because they are printed as numbers most buyers have no reference for — so we fixed them rather than leaving them to chance.

The full standard is set out on our standards page. Build a ring to it in the ring builder, or book a consultation and we will walk through a report with you.

Frequently asked questions

What is the table of a diamond?

The table is the large, flat facet on the very top of the stone. It is the main window through which light enters and returns, and its width relative to the diamond's total width is the table percentage.

What is an ideal cut diamond?

One whose proportions — especially table and depth percentage — fall in the narrow range that returns the most light to the eye. Diamond's critical angle of about 24.4 degrees is what makes that range narrow: outside it, light refracts out of the stone instead of reflecting back.

What is a good table and depth percentage?

It depends entirely on shape. For a round brilliant, roughly 54–57% table and 59–62.5% depth. An emerald cut needs a much wider table at 62–66% with 61–67% depth. Applying round-brilliant figures to a fancy shape is the most common mistake.

Is there one ideal table and depth for all diamond shapes?

No. Facet architecture differs by shape, so step cuts such as emerald need a wider table than brilliant cuts. Any source quoting a single universal ideal is describing round brilliants only.

Does table and depth affect sparkle?

Hugely. They are the core of the cut, and cut determines how much light a diamond returns — more than colour, clarity or carat weight do.

What happens if depth is too deep or too shallow?

Too deep and light reflects out the sides rather than up, so the stone reads dark at the middle and hides weight below the girdle, wearing smaller than its carat. Too shallow and light escapes through the bottom, leaving a pale window under the table and a watery, glassy look.

Can a diamond have an Excellent cut grade but not ideal proportions?

Yes. The Excellent grade spans a wider range of proportions than the ideal window, so a stone can earn it while sitting at the edge of the band. This is why Montare requires ideal table and depth in addition to a Triple Excellent grade.

Where do I find table and depth on a diamond certificate?

In the proportions diagram, usually printed beside the plotted stone and expressed as percentages. The separate measurements line gives the actual millimetre dimensions, which is how you spot a stone cut deep to preserve weight.

Cut to ideal, every time. Build a ring in the ring builder or book a consultation.