"A diamond is cut too shallow with respect to its width will allow too much light to pass straight through the diamond and leaving little light to reflect back to the viewer." This description explains that a shallow cut causes light leakage through the bottom rather than returning to the observer’s eye, resulting in reduced brightness.

"Diamond brilliance refers to the white light reflected back to the eye from both the surface and the internal facets of a diamond."[4] "Entry, Reflection, and Refraction: Light must enter at the right angles to maximize brightness."[4] "Critical Angles: If cut too shallow, light leaks through the bottom; if cut too deep, light escapes through the sides."[4] "Shallow Cut: Appears wider across the surface but causes light to leak through the bottom, reducing brilliance."[4] "Deep Cut: Retains weight in the pavilion, but much of the light escapes through the sides, making the diamond appear smaller and darker."[4]

"Generally, crown angles from 32 to 36 degrees produce attractive, bright, and fiery diamonds if combined with the right table percentage and pavilion angle." It notes that "Subtle pavilion angle differences can have a significant impact on the diamond’s face-up appearance. For instance, steep pavilion angles can produce dark areas under the diamond’s table." This shows how non‑optimal pavilion angles (too steep) reduce brightness under the table, while proper crown/table/pavilion combinations maximize light return through the crown rather than leaking out the bottom.

"Here’s what happens when light hits a diamond: Reflection: Some light bounces right off the top of the diamond through the table facet. The rest of the light goes deeper inside the diamond and reflects off the internal walls."[1] "The depth of a diamond, measured from the table (top) to the culet (bottom), affects how well it reflects light."[1] "A pavilion that is cut too deep or too shallow can cause light to escape from the bottom or sides, reducing the diamond’s sparkle."[1] "Its [the pavilion's] purpose is to reflect light up through the top of the diamond, contributing significantly to the stone’s brilliance."[1]

"The amount of light returned by the stone is affected by the pavilion's depth. If the pavilion is either too shallow or too deep, light escapes and the diamond appears dull and less dazzling. Poorly cut pavilion facets cause light leakage, reducing the sparkle." It further explains: "A too shallow pavilion creates a ‘fisheye’ effect… Instead, [light] jumps off the crown facets and exits the diamond. A pavilion that is too deep has a wider pavilion angle, making the light bounce off a different direction and escape the pavilion facets rather than returning back to the table. The table appears dark at this point because of light leakage at the pavilion."

"Light Performance: The cut of a diamond determines how well it interacts with light. When light enters a diamond, it is refracted, reflected, and dispersed. A well-cut diamond will reflect most of the light out of the top of the stone, creating exceptional brilliance. In contrast, a poorly cut diamond allows light to escape through the sides or bottom, resulting in less sparkle."[6] "For example, if the cut is too shallow, light escapes from the bottom, causing the diamond to lose its brilliance. If the cut is too deep, light escapes from the sides, also reducing sparkle."[6] "Ideal proportions ensure that light is reflected within the diamond and exits through the top, maximizing brilliance."[6]

"Pavilion facets are responsible for reflecting light internally and back through the crown. When these facets are cut at the correct angles, light entering the diamond bounces between them before and returning through the crown back to the eye... If the pavilion is too deep or too shallow, light can escape through the bottom of the diamond, reducing brilliance." This explains that incorrect shallow or deep pavilion angles cause light leakage through the bottom, reducing brightness, and that pavilion facets must return light through the crown rather than letting it leak.

When light strikes a diamond, approximately 20% immediately reflects off the surface (as glare). Of the 80% that enters, a portion will escape through the bottom of the diamond (where the observer cannot appreciate it). A well proportioned diamond will have each facet properly placed and angled so as to maximize the amount of light that reflects back out of the crown (top) of the diamond, to the eye of the observer. In the diagram below, three common light patterns are shown. When light meets any facet of a diamond, it will either reflect (bounce back) or refract (bend while passing through the facet).

Discussing a shallow diamond: "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." On a deep diamond: "the deep diamond on the right… 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."

"Brilliance refers to how bright the diamond looks. A quality diamond cut should reflect lots of white light. If the cut is wrong, uneven, too deep or too shallow, it appears dull."[8] "Shallow cut: When a diamond is cut too shallow, it doesn’t sparkle as brilliantly. These diamonds, often called ‘spread’, lack depth compared to ideal cuts. Most of the light that enters escapes from the bottom, leading to a lacklustre appearance."[8] "Deep cut: When a diamond is cut too deep, it traps the light. The light hits the pavilion (top) at a sharper angle and is forced to pass through the bottom of the diamond. The result is less sparkle and liveliness, and lower brightness."[8] "Table: The table, the diamond’s largest facet, is the flat top. It’s the primary entry and exit point for light, responsible for the diamond’s sparkle. The table’s size affects the diamond’s light intake and overall performance."[8]

"The table is the largest facet of a stone. The table refracts light rays and directs them to additional facets as they pass through the stone’s... A diamond table that is too large may be hindered when dispersing light... The pavilion plays an essential role in a diamond’s light-reflecting properties. An excessively deep or shallow diamond can cause light to escape from the bottom and sides, reducing its sparkle... If a stone’s cut is too shallow, the light will enter the diamond only to escape through the pavilion. Shallow cut diamonds are unable to reflect light for this reason and lack brilliance." This shows that overly shallow or deep cuts produce light leakage through the bottom and sides and that shallow cuts generally lack brilliance despite the table being able to refract and direct light.

"Round brilliants achieve optimal light return with pavilion angles between 40.6 and 41.8 degrees… Deviations create conditions where light rays exceed critical thresholds—resulting in leakage rather than internal reflection." It explains "Diamond windowing occurs when light passes through a diamond without reflecting back to the observer's eye, creating dull, transparent areas within the stone… When pavilion main facets are cut too shallow (typically below 40% depth ratio), light escapes through the bottom instead of bouncing back through the crown." This is described as failure of total internal reflection due to the critical angle of diamond.

"Shallow cut diamonds have a shallow depth relative to their diameter. Light entering the diamond may exit through the pavilion facets rather than being reflected back to the viewer’s eye, resulting in less brilliance and sparkle."[3] "Shallow cut diamonds may appear larger for their carat weight, but they can lack the optimal play of light and may look less vibrant."[3] "Deep cut diamonds have a greater depth relative to their diameter. Light entering the diamond may escape through the sides or bottom, resulting in reduced brilliance and a potential dark appearance in the center."[3] "An ideal cut diamond is precisely cut to maximize brilliance and sparkle. The proportions are balanced to allow light to reflect internally and return through the top of the diamond (crown), creating a dazzling appearance."[3]

"A diamond’s table is its largest facet. The table is the top flat shiny surface of the stone. If the table is too wide, it overwhelms the body of the diamond. However, a table that is too small won’t pull in and reflect enough light... If a diamond is too shallow in depth, light will enter the diamond but escape through the bottom of the diamond. This means you won’t have the white ‘flash’ that many diamonds exhibit when moved under light, and the diamond will appear darker... Light is also lost when a diamond is cut too deep... causing light to reflect out the sides of the diamond." This indicates that shallow-cut diamonds lose light through the bottom and appear darker, and deep-cut diamonds lose light through the sides, rather than appearing brighter, even though the table facet is responsible for pulling in and reflecting light.

When a diamond is cut too shallow, light leaks out of the bottom, brilliance is lost and the diamond appears watery, glassy and dark. A diamond with these characteristics is referred to as a "fisheye". When a diamond is cut too deep, light leaks out of the sides, brilliance is lost and the center of the diamond will appear to be dark. A diamond with these characteristics is referred to as a "nailhead". A pavilion depth that’s too shallow or too deep will allow light to escape through the sides or the bottom of the stone. A well-cut diamond will direct more light through the crown.

"When proportions fall within these ranges, light entering through the crown reflects off the pavilion facets and returns to the viewer's eye. Move outside these parameters, and light escapes through the sides or bottom, creating dead zones that reduce brilliance." It notes that "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." It further states: "Steep pavilion angles produce dark areas under the diamond's table, while shallow pavilion angles make the diamond appear foggy."

"Shallow cuts let light leak straight through the bottom, leaving the stone looking glassy and lifeless."[9] "Deep cuts push light out the sides, making the center look dark and less brilliant."[9] "When cut to ideal proportions, a diamond bounces light around its interior and sends it back out through the top, creating the sparkle we love."[9]

When light strikes a diamond, approximately 20% immediately reflects off the surface (as glare). Of the light that enters, poorly proportioned stones allow more light to escape through the pavilion where it cannot be seen, while well-cut diamonds return more light through the crown. Shallow‑cut diamonds typically show a fisheye effect and appear glassy and less brilliant because of increased light leakage. Deep‑cut diamonds often show a dark center (nailhead) as light leaks through the sides instead of returning to the viewer.

The video explains that when crown and pavilion angles are cut too steep, the result is a "poor light performing Steep Deep cut diamond." In the analysis: "as you can see the steep deep cut has severe light leakages under the table which causes the middle of the diamond to have a very dark and dull appearance… overall it will make the diamond look much dimmer and lifeless." The presenter recommends pavilion angles "to be under 41 degrees" and notes that steep deep cuts show a "ring of death" under the table due to light leakage rather than light returning to the viewer.

"When light enters a gemstone, it is refracted and reflected. An ideal cut gemstone can return most of the light from the top, creating brilliant sparkle and brightness. If the cutting proportions are improper, light will escape from the sides or bottom, causing the gemstone to appear dull... Deep cut gemstones do not effectively reflect light from the gemstone’s surface; light is mainly concentrated at the bottom. This results in reduced fire and brightness... Due to insufficient depth, light entering the gemstone cannot fully reflect inside, often escaping directly from the bottom. This makes the brightness and fire effect of the gemstone appear dull." Though referring to gemstones generally, this explains that both shallow and deep cuts cause light leakage (bottom or sides) and poorer brightness, while ideal cuts maximize light return from the top.

A contributor explains that a pavilion angle of 41.4° and pavilion depth of 44% "is not likely to produce a high volume of light return" and that "the deeper pavilion depth is likely to make the middle of the diamond look dark due to the increased chance of light leakage." They describe a "critical tipping point" around 43.5% pavilion depth where light "begins not to strike fully off the lower half of the diamond," implying more light escapes through the pavilion instead of returning through the crown.

"Diamonds that are well-cut maximize their ability to reflect light from the top, making it very bright or attractive. A deep cut or very shallow cut will allow light to escape from the sides or bottom, reducing the diamond's brilliance." This summary indicates that both very shallow and deep cuts permit light escape rather than maximizing brightness, and that well-cut diamonds reflect light back through the top.

Ideal and Super Ideal Diamonds like the A CUT ABOVE® achieve a perfect balance of diamond light performance properties that give them mesmerizing beauty. Maximizing light return alone is not the only goal; contrast, dispersion and scintillation also contribute to perceived beauty. Diamonds with sub‑optimal proportions may leak light either through the pavilion or at low angles, reducing overall brightness even if some surface reflections remain.

The notably high crown and pavilion angles will negatively affect light performance. When both angles are high, the result is a “steep/deep” cut, as shown here—definitely not ideal. The stone leaks excessive white light and returns mostly low‑angle green light, reducing overall brightness. A well‑cut diamond shows the opposite ASET pattern: more red, less green, and minimal white, corresponding to stronger light return and brightness.

"If you twirl the diamond in the light would you expect to see a reflection of the full table at times?"[2] "Responses in the discussion explain that under certain viewing angles you can see reflections of the table facet in other facets, but that this is a geometric effect of the facets and not necessarily an indicator of superior brilliance."[2]

Cut is the dominant factor in how much a diamond sparkles. A well-cut SI1 will outshine a poorly cut VVS2 in virtually every lighting condition. Clarity affects sparkle only when inclusions are large enough or centrally located enough to interrupt light return, which is uncommon at VS2 and above. Lower clarity diamonds with visible inclusions can appear cloudy or dull, while higher clarity stones allow more light to pass through, enhancing sparkle and brilliance.

In lighting tests comparing two diamonds with the same GIA grades, the diamond suffering from weaker light return shows dark and dull areas in the middle caused by light leakages. When a diamond suffers from weaker light return or light leakages it will get these dark areas in the middle which makes the entire diamond look very bland. The super ideal cut diamond, designed for maximum light return, appears much brighter even at further distances and stands out more under white light conditions.