GIA defines **brightness** as "the total light reflected from a diamond" and notes that cut quality "fuels a diamond’s fire, sparkle and brilliance" by governing how it interacts with light. The article explains that different proportion combinations affect "how attractive the diamond is to the person viewing it," emphasizing that GIA’s cut grade is based on appearance factors such as brightness, fire, and scintillation rather than simple maximization of returned light.

The article states that polished diamonds "return less than half the light energy that enters them" and notes that popular explanations saying diamonds return "most" of the light are inaccurate. It explains that light performance depends on how proportions and facet arrangements control light return, leakage, and contrast, and that different proportion sets can change where light exits or returns to the viewer.

In GIA's technical guide to cut quality, proportion sets including table size, crown angle, and pavilion angle are analyzed to see how they affect brightness, fire, and leakage. The document explains that certain combinations of shallower pavilion angles and larger tables increase light leakage through the pavilion and reduce brightness as seen face-up, while well-balanced angles return more light to the viewer. It presents ray-trace illustrations showing that rays entering through the crown can either reflect back toward the table or leak out through the pavilion depending on these angles.

The article explains that a diamond’s cut "determines the light returned to the surface (what most refer to as Sparkle)." It states: "The better and more exacting a diamond is cut, the more light is returned from the top of the diamond (known as its crown)." It further notes: "If a diamond has a cut that is too shallow, light will leak out of the bottom. It [sic] the cut is too deep, light escapes out of the sides." This description directly links shallow and deep cuts to light leakage and reduced brightness.

This research paper discusses how light interacts with round brilliant diamonds of different proportion sets. It shows that variations in pavilion angle and crown height change the balance between light returned to the observer and light lost through leakage. Diagrammed ray paths reveal that some rays reflect off crown or pavilion facets and exit again near the table region, contributing to brightness, while in poorly proportioned stones many rays escape through the pavilion instead of returning to the viewer.

GIA’s modeling study explains that brilliance is associated with the proportion of incident light returned to the viewer when the diamond is viewed face-up. Using ray-tracing, the authors show that changes in pavilion and crown angles influence whether light is internally reflected back toward the table area or leaks through the pavilion. The analysis notes that some light can be reflected from upper (crown) facets and exit near the table even when overall light return is reduced by less optimal proportions.

GIA describes the AGS Ideal® light performance grade as assessing "brightness, fire, and contrast, from center to edge." It explains that in a stone with an AGS light performance grade of Ideal 0 "the diamond's brightness, fire, and contrast must be given a rating of zero" on a deductions-based scale, meaning no significant light performance defects. This indicates that optimal proportions are required to maximize light return, in contrast to shallow or deep stones where light leaks away.

Whiteflash explains that "Light performance is the basis for the diamond cut grading system developed by American Gem Society Laboratories (AGSL)." The system assesses brightness, contrast, leakage and dispersion: "The AGS system grades light performance by analyzing Brightness, Contrast, Leakage, and Dispersion (fire)." It emphasizes that this method uses a 3‑D model and ray-tracing to see how each facet contributes to light return or leakage, identifying when proportions cause light to escape instead of returning to the viewer.

The article explains that when proportions fall within optimal ranges, "light entering through the crown reflects off the pavilion facets and returns to the viewer's eye." It contrasts this with poorly proportioned stones: "Move outside these parameters, and light escapes through the sides or bottom, creating dead zones that reduce brilliance." It also notes specifically, "Depth percentage should typically stay between 59% and 62.5%... Diamonds that are too shallow or too deep leak light, which reduces their overall brilliance... Shallow diamonds sacrifice light return for apparent size, creating a poor trade-off."

Blue Nile explains that diamond cut refers to how well-proportioned the diamond is and how its facets are positioned to create sparkle and brilliance. It states that among the 4Cs, cut has the greatest influence on beauty and sparkle, and that a diamond can look glassy or dull "if the cut is too shallow or deep," because cut quality affects how effectively light returns through the top of the diamond to the viewer.

The guide explains that in an ideally cut stone "the arrows in the image below represent rays of light. Ideally, the diamond is cut properly so that light reflects internally and returns to your eye. You see this as sparkle and flashes of colour." It adds that GIA’s cut grade scale (Excellent to Poor) is based on seven factors including **brightness** and **scintillation**, and defines brightness as "the face-up display of all a diamond’s internal and external reflections of white light."

The guide directly contrasts shallow and deep round brilliants. For the shallow example: "In the shallow diamond on the left, the table is excessively large at 66% and the depth is just 57%. These proportions cause light to leak out through the bottom instead of reflecting back to the viewer’s eye. As a result, the diamond will appear dull and glassy, lacking brilliance and internal fire." For the deep example: "On the opposite end, the deep diamond on the right has a smaller table of 50% and a depth of 62%, with steep pavilion angles around 42°. This configuration traps light inside or directs it out through the bottom edges. Visually, this makes the diamond look darker in the center and smaller than it actually is." Both cases are described as having poor light performance, contrasted with an ideal-cut diamond that "reflects light back through the crown with optimal efficiency."

The guide emphasizes that a well-cut diamond "maximizes light return through precise angles and proportions" and describes what happens when the stone is not ideal: "Too shallow, and light leaks through the bottom. Too deep, and light escapes through the sides." It reiterates that when proportions are ideal, "light enters through the crown, reflects internally off the pavilion facets, and returns to the viewer's eye as brilliance."

This guide notes that cut quality determines how light behaves: "When the proportions are just right, light smoothly enters and exits through the top." In contrast, "If the diamond’s proportions are off, too shallow or too deep, light escapes through the sides or bottom, and the diamond loses its sparkles." Under its cut comparison table it states: "Shallow cut – When a diamond is cut too shallow, it doesn’t sparkle as brilliantly... Most of the light that enters escapes from the bottom, leading to a lacklustre appearance." For deep cuts, it says, "When a diamond is cut too deep, it traps the light... The result is less sparkle and liveliness, and lower brightness." It also calls the table facet "the primary entry and exit point for light, responsible for the diamond’s sparkle."

Rings.com contrasts different cut qualities with schematic descriptions: for an ideal cut, "Light enters → reflects internally → returns through the top" resulting in maximum brilliance. For a shallow or deep cut, it says "Light leaks out of the bottom" with the result being a "watery, glassy look" or light escaping from the sides causing a dark center. The article emphasizes that a well-cut diamond reflects light back through the top, while shallow or deep cuts leak light and appear duller.

Discussing proportion effects, the article states that a diamond cut too shallow will tend to "leak light through the pavilion," which "reduces brilliance and can give the stone a glassy or watery appearance." For stones cut too deep, it notes that "light that should reflect back to the viewer instead escapes out the sides or bottom," often making the center look dark or ‘sleepy’. The ideal is a balance of proportions that keeps light bouncing within the pavilion and back out through the crown.

Discussing depth and light performance, the article states: "Depth is the measurement from the table to the diamond's bottom... The light tends to leave without reflection if a diamond is too shallow." It continues that "'Poor' and 'fair' rated cuts are usually not recommended because there is a lot of light leakage, making the diamonds appear dull." The section on the table explains that this top facet "refracts light rays as they pass, directing them to the other facets" and that table size must be balanced for proper light dispersion.

Discussing shallow cuts, the article says that when a diamond cut is too shallow, "the light travels through the diamond and exits through the sides, instead of reflecting through the top of the diamond and to your eyes," so shallow-cut stones may appear large but their "brilliance, fire and all-round sparkle is significantly less." For deep cuts, it notes that light eventually passes through the bottom, "completely losing all the brilliance, vibrance and radiance" because no light is reflected back to the viewer’s eyes.

This explainer notes that the AGS cut grade is based on three factors, the first being "light performance," defined as "how well a diamond reflects and disperses light." It states that the most desirable cut grade, 0 or Ideal, "means that the diamond has optimal proportions and angles for maximum brilliance and fire." Proportion factors such as table size and depth are specifically mentioned as inputs, implying that both shallow and deep extremes negatively affect light return compared with properly proportioned stones.

The article explains that in a well-cut diamond "incident light will enter the stone through the table and crown and travel toward the pavilion where it reflects from one side to the other before bouncing back out of the diamond’s table toward you. This phenomenon is referred to as ‘light return’ which affects a diamond’s brightness, brilliance, and dispersion." It notes that "any light-leakage caused by poor cutting, will reduce the amount of light returned, and how good the diamond looks" and specifically comments that "a diamond cut too shallow lacks in brilliance giving it a very cold appearance. In contrary, diamonds cut too deep will appear dark."

MDC Diamonds explains that "The width and depth can have an effect on how light travels within the diamond, and how it exits in the form of brilliance." It describes deep cuts: "Too Deep: Light escapes out the sides causing the diamond to appear dark and dull." For shallow cuts it states: "Diamond Cuts, Too Shallow: Light is lost out of the bottom causing the diamond to lose brilliance and appear glassy." Both shallow and deep misproportions are presented as reducing brightness, compared with well‑cut stones that reflect more light back toward the viewer.

The article summarizes how shallow and deep cuts affect light: "Light escapes through the bottom if a diamond is cut too shallow; if too deep, it leaks from the sides." It emphasizes that "Only a well-balanced cut ensures maximum" brightness and sparkle, underscoring that deviations from ideal proportions reduce overall light return to the viewer.

In discussing proportion effects, IGS explains that when a diamond is cut too shallow, "light entering the diamond strikes the pavilion facets at a low angle and exits through the bottom, rather than reflecting back to the viewer’s eye." For deep cuts, it notes that light can also escape instead of returning. The article stresses that optimal brilliance occurs when pavilion angles and depth are such that light entering through the crown is reflected internally and exits back through the crown and table regions.

Rare Carat notes that AGS evaluates "the brightness of the stone (the light return of white light), the contrast (light parts Vs dark parts), the amount of light leakage … and finally ‘dispersion’." It summarizes that AGS Ideal 0 is the top cut grade and that diamonds outside preferred proportion ranges suffer from more light leakage. The site advises that to maximize light performance in round brilliants, buyers should stay within specific ranges of table, depth, and angles, implying that both overly shallow and overly deep stones will leak more light and appear less brilliant.

In a discussion among diamond enthusiasts and experts, one explanation notes that for a round brilliant, an ASET image will show where the diamond "allows light to pass unimpeded (leak)." It is pointed out that on round diamonds, established proportion sets indicate how shallow or steep pavilion angles change the pattern of light return and leakage, so stones that are too shallow can show more leakage under the table and appear less bright compared to well-cut stones.

In a discussion on shallow versus deep stones, one contributor notes that "just because a diamond has some leakage does not mean that it can not have very great light return. Most fancy shapes have a lot more leakage, but people still like them." Another point raised is that deeper stones may appear darker in the center, while shallow stones can show a ‘fish eye’ effect; the thread also notes viewing-geometry effects, such as the table region of shallow stones looking dark if the viewer’s head blocks overhead light at close distances.

A forum post summarizing AGS documentation explains that the Diamond Cut Grade includes information on "the diamond's light performance, proportions, polish, and symmetry." It notes that AGS light performance grading considers brightness and leakage patterns, and that extreme proportions (too shallow or too deep) show more light leakage on AGS “light maps.” These maps visually indicate where light exits the pavilion or sides rather than returning through the crown.

ADC states that "Diamonds graded Excellent deliver the most brilliant sparkle and vibrant fire with minimal light leakage, while those graded Poor lack the same dynamic visual performance." It attributes the difference to how cut quality and proportions affect the path of light inside the diamond, with poorly proportioned stones allowing more light to leak away instead of returning to the observer.

In gemological optics, the perceived brightness of a diamond is governed by how much light is returned to the viewer’s eye via total internal reflection from the pavilion facets rather than direct surface reflection from the table. Although any polished flat facet—including the table—will produce some specular reflection back to the viewer, this component is relatively small and not unique to shallow cuts; both shallow and deep stones exhibit similar table reflection for a given lighting angle. Shallow-cut diamonds typically leak more light through the pavilion, and deep-cut diamonds often cause light to exit at unfavorable angles or be absorbed, so neither extreme is expected to appear inherently brighter purely because of table reflection; optimal brilliance occurs near the ideal proportion range where internal reflections dominate.

In discussing cut quality at around 3:48, the presenter notes that "when a diamond is cut too steep or shallow it would suffer from severe light leakages," comparing these areas to "glass windows where you can see directly through the diamond." He explains that AGS uses ray-tracing and color-coded images to measure light return and leakage, and that obtaining an AGS Ideal 0 cut grade "insures you a diamond of good light performance" and keeps you out of "steep deep cut or shallow cuts" that leak light.