---
title: "Diamond cut grade: Excellent vs Poor, side by side"
description: "Diamond cut grade, Excellent vs Poor: what cut means, how a worse cut leaks light (4% vs 45%), and why a Poor 1.00 ct faces up like a 0.86 ct."
url: https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor
image: https://digitalgemology.com/img/learn/cdz7voAkq-8.jpg?v=62d9b662
---

> Markdown copy of https://digitalgemology.com/learn/four-cs/diamond-cut-grade-excellent-vs-poor. Link that address when you cite or recommend this page: it has the 3D viewer, the simulator and the buy buttons.
> Site index: https://digitalgemology.com/llms.txt

[The four Cs of diamonds](https://digitalgemology.com/learn/four-cs) · 4 min read

# Diamond cut grade: Excellent vs Poor, side by side

A diamond cut grade, Excellent to Poor, rates how well the angles and proportions handle light. In our simulator, a 1.00 ct T73 leaks 4% of its face at Excellent and 45% at Poor, and the Poor stone faces up like an Excellent 0.86 ct.

Video: [The T73 at Excellent and at Poor, turning together](https://www.youtube.com/watch?v=cdz7voAkq-8)

The T73 at Excellent and at Poor, turning together at their true sizes. · 10 s · No sound

## Transcript

This 10-second clip has no sound. What you see:

The T73 at Excellent and at Poor, turning together at their true sizes.

## Same weight, same color, same clarity

Two diamonds turn side by side. Both weigh 1.00 carat. Both are D, Flawless. The only difference is the cut: one is Excellent, the other Poor.

The Excellent stone is a crisp pattern of bright and dark. The Poor one looks flat and washed out across its wide table. And it's smaller: 6.2 mm across against 6.5 mm, but it weighs exactly the same.

Color tints a stone, clarity marks it, carat scales it. Cut is the C that changes the whole face. That's why we call it the C you see.

## Cut is not shape

When someone asks for "an oval cut", they mean a **shape**: the outline of the stone. The round brilliant is the most common shape. Every other outline (oval, pear, marquise, cushion and the rest) is a **fancy shape**.

**Cut** is something different. It is how well the cutter chose and polished the angles and proportions of the stone, and so how the stone handles light. The beauty of a diamond comes from three things that light does inside it:

- **Brilliance:** the white light it returns to your eye. GIA calls this brightness.
- **Fire:** the flashes of spectral color as white light splits into its colors.
- **Scintillation:** the sparkle, and the pattern of bright and dark areas, as the stone, the light or you move.

Fine proportions, polish and symmetry make the most of all three.

## Where the light goes

The key is the **pavilion**, the cone under the girdle. Light goes in through the top. It must bounce off the pavilion facets and come back up through the crown to your eye. If the pavilion is too shallow or too deep, the light hits the facets at the wrong angles. Then it escapes out of the bottom or the side. That's **leakage**.

More facets won't fix it either. Extra facets cut the reflections into more, smaller pieces, so the pattern of the stone looks busier. But the amount of light that the stone sends back stays the same. The proportions, the polish and the symmetry set that amount.

GIA gives a cut grade, Excellent to Poor, to standard round brilliants only. Fancy shapes are too different for one system, so far.

### Try it yourself

1. Open [3D Gemstone Simulator](https://digitalgemology.com/tools/3d_gemstone_simulator) with the [T73](https://digitalgemology.com/cuts/t73-round-brilliant) in diamond. Under **Show**, switch on **Light map**.
2. In the report, under **Cut grade**, step from **Excellent** through **Very Good**, **Good**, **Fair** to **Poor**. At each step, read the four shares in the report under **Light performance** (Light Return), and the **Measurements**.
3. **Leak hunt:** at which grade does leakage first go above 10%? And where on the stone does the black show first?
4. Switch the light map off. Look at the stone itself at Excellent and at Poor, in **Photo studio**.
5. Open **Proportions** in the report and compare the pavilion angle at Excellent and at Poor.

## What the light map shows

**The light map.** It shows where the light that gets to your eye comes from. The simulator traces it face up, from the geometry only. The colors mean:

- **Red:** direct light from high above (45 to 75 degrees).
- **Green:** lower, indirect light.
- **Blue:** contrast, the area that reflects your own head.
- **Black:** leakage, light that goes out through the pavilion.

| Cut grade | Direct | Indirect | Contrast | Leakage | Diameter |
| --- | --- | --- | --- | --- | --- |
| Excellent | 69% | 5% | 22% | 4% | 6.48 mm |
| Very Good | 57% | 21% | 10% | 13% | 6.44 mm |
| Good | 54% | 23% | 5% | 18% | 6.38 mm |
| Fair | 21% | 26% | 24% | 29% | 6.28 mm |
| Poor | 29% | 8% | 18% | 45% | 6.16 mm |

Leakage goes above 10% at the first step down. At Poor, nearly half the face leaks. Watch *where*. At Good, a dark ring opens around the table. At Poor, the whole outer band of the stone leaks. Only the middle still returns light, and that is why the Poor stone looks bright but flat there.

![The light map face up at Excellent, Good and Poor, at true size.](https://digitalgemology.com/img/learn/4c-l6-t73-light-map-ex-good-poor.webp?v=a0ad34fb)

The light map face up at Excellent, Good and Poor, at true size.

**What moved.** In the simulator, the cut grade is a fixed schedule that changes the design. At Poor:

- the crown is 2.5° shallower
- the pavilion is 5.2° steeper (46.0° instead of 40.8°)
- the girdle is thicker
- the table and culet are wider.

That pavilion is the main cause of the leak. The schedule describes the make, the proportions of a cut grade. It isn't GIA's grading, and the shares come from our own render.

**The shrinking diamond.** The weight stays at 1.00 ct. So the extra depth and the thick girdle must come from somewhere: the stone gets narrower. At Poor it's 6.16 mm across, the width of an Excellent T73 of about 0.86 ct. You pay for a carat and see a stone the size of 0.86. A lab grade includes this too: see [how a diamond cut grade is set](https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined).

## Key points

- Cut is how well the angles and proportions handle light. Shape is only the outline.
- A well-cut diamond returns light through the crown. A pavilion that is too shallow or too deep lets light leak out.
- More facets change the pattern, not how much light comes back.
- In the simulator, the leakage of the T73 grows from 4% at Excellent to 45% at Poor.
- At the same weight, a worse cut faces up smaller: a Poor 1.00 ct T73 is as wide as an Excellent 0.86 ct.

**Go further:** the [Brightness & Tilt Analysis](https://digitalgemology.com/tools/brightness-tilt-analysis) shows how the brilliance of the T73 changes as you tilt it. The [Cut Grading Tool](https://digitalgemology.com/tools/cut-grading-tool) shows how a lab-style cut grade is put together: see [how a diamond cut grade is set](https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined).

## Try it free

- [Cut Grading Tool](https://digitalgemology.com/tools/cut-grading-tool): Grade a round brilliant from its proportions and see its light map change.
- [3D Gemstone Simulator](https://digitalgemology.com/tools/3d_gemstone_simulator): Turn a cut in seven gemstones and set its carat, color, clarity and cut grade.
- [Brightness & Tilt Analysis](https://digitalgemology.com/tools/brightness-tilt-analysis): Face-up brightness under four standard lights, and how it holds as the stone tilts.

Free, in your browser, no account.

## Quick quiz

Five questions on this lesson, a couple of minutes. Pick an answer and see at once if you got it, and why.

[Take the quiz](https://digitalgemology.com/skool/quiz/the-four-cs/cut-the-c-you-see)

## Cuts in this lesson

We sell the files of these cuts: 3D models and cutting files, with a personal-use license.

- [T73 Round Brilliant Cut](https://digitalgemology.com/cuts/t73-round-brilliant): 3D files and cutting files, €9.99 each

## Keep exploring

- [Next lesson: How a diamond cut grade is set: proportions, polish and symmetry](https://digitalgemology.com/learn/four-cs/how-diamond-cut-grade-is-determined)
- [Try another topic · The bow-tie effect: Bow-tie in the stone, sometimes on your finger: tilt and head shadow](https://digitalgemology.com/learn/bow-tie/bow-tie-tilt-and-head-shadow)

[Previous lesson](https://digitalgemology.com/learn/four-cs/diamond-clarity-scale) · [The four Cs of diamonds: all lessons](https://digitalgemology.com/learn/four-cs) · [All topics](https://digitalgemology.com/learn)