Diamond Cut

When purchasing a diamond, many buyers focus on carat weight or clarity, but the most important factor influencing a diamond’s beauty is its cut. The cut determines how well a diamond interacts with light, creating the brilliance, fire, and scintillation that give a diamond its sparkle. A poorly cut diamond can appear dull, even if it has high clarity and color grades.

Cut vs. Shape: Understanding the Difference

A common misconception is that diamond cut and diamond shape are the same thing. Diamond shape refers to the overall form of the stone (round, princess, oval, etc.), while diamond cut refers to how well the facets are proportioned and polished to maximize light return.

How Cut Affects Sparkle

A well-cut diamond reflects light internally from one facet to another before returning it to the eye. This maximizes its brilliance and fire. A poorly cut diamond can leak light, resulting in reduced sparkle and a lifeless appearance. Cut is the single biggest factor that determines how much a diamond will shine.

AGS Merger with GIA: What It Means for Cut Grading

Historically, both the American Gem Society (AGS) and the Gemological Institute of America (GIA) graded diamond cut quality. AGS was known for its Ideal 0 grading system, while GIA used Excellent, Very Good, Good, Fair, and Poor grades. In 2022, AGS was acquired by GIA, and now all diamonds are graded under GIA’s system. This means that when shopping for the best-cut diamonds, buyers should focus on GIA Excellent Cut or AGS Ideal 0 Cut, as these represent the top tier of precision cutting.

Diamond Anatomy: The Parts That Influence Light Performance

To understand diamond cut, it’s important to know the different parts of a diamond and how they impact light performance. Each facet plays a role in how light is reflected, refracted, and dispersed.

Key Parts of a Diamond

• Table: The largest, flat facet on the top of the diamond. Affects how much light enters the diamond and contributes to overall brightness.
• Crown: The upper portion of the diamond between the table and girdle. Influences the dispersion of light (fire) and overall sparkle.
• Girdle: The outer edge that separates the crown from the pavilion. Can be thin, medium, or thick, affecting durability and setting options.
• Pavilion: The lower section of the diamond beneath the girdle. The pavilion angle determines how well light reflects back through the top of the stone.
• Culet: The small facet at the very bottom of the diamond. Ideally, it should be either “None” or “Very Small” to prevent light leakage.

How Diamond Anatomy Affects Sparkle

Each of these elements must be precisely proportioned for a diamond to exhibit maximum brilliance. A table that is too large or too small can impact light reflection. A pavilion that is too deep or shallow will cause light leakage, reducing sparkle.

Understanding Diamond Cut Grades (GIA System)

Not all diamonds are cut equally, and grading organizations assess their proportions, symmetry, and polish to determine quality. The Gemological Institute of America (GIA), which now includes AGS standards, assigns diamonds one of five cut grades:

• Excellent – Maximum brilliance and fire; ideal proportions.
• Very Good – Slightly less light return but still highly brilliant.
• Good – Noticeable light leakage, reducing sparkle.
• Fair – Significant light leakage, leading to a dull appearance.
• Poor – Minimal brilliance due to excessive light loss.

GIA vs. AGS Cut Grading

Before merging with GIA, AGS used a numeric scale (0 to 10), where 0 was Ideal (equivalent to GIA Excellent) and 10 was Poor. Now, all diamonds are assessed using the GIA system, though many high-end jewelers still reference AGS Ideal 0 when discussing superior cut quality.

For buyers seeking the best brilliance, GIA Excellent or AGS Ideal 0 diamonds are the top choices.

Factors That Influence Cut Grade

GIA evaluates a diamond’s cut based on:

• Proportions – How well the facets are arranged to optimize light performance.
• Symmetry – The precision of facet alignment.
• Polish – The smoothness of a diamond’s surface.
• Light Return – How effectively the diamond reflects and refracts light.

Ideal Proportions for a Brilliant Cut Diamond

For round brilliant diamonds, specific proportions maximize light performance. The following table outlines the ideal ranges:

Diamond Proportion	Ideal Range
Table %	53% – 58%
Depth %	59% – 62.5%
Crown Angle	34° – 35°
Pavilion Angle	40.6° – 41°
Girdle Thickness	Thin to Medium
Culet	None to Very Small

Why Proportions Matter

While proportions are an important guideline for assessing diamond cut, two diamonds with the same proportions can have varying cut quality. This is because light performance is influenced by factors beyond just proportions, such as symmetry, facet precision, and polish.

Cut Quality for Fancy-Shaped Diamonds

Unlike round brilliant diamonds, fancy-shaped diamonds (such as princess, oval, cushion, and emerald cuts) do not have standardized cut grading. Instead, their brilliance depends on facet arrangement, symmetry, and depth percentages.

Best Cut Guidelines for Popular Shapes:

• Princess Cut: Depth 67%-75%, Table 65%-72%.
• Oval Cut: Avoid deep stones (>66%) to prevent the bow-tie effect.
• Cushion Cut: Modern vs. antique cushions have different brilliance patterns.
• Emerald & Asscher Cut: Step cuts rely more on clarity and symmetry than brilliance.

For fancy shapes, light performance images are even more crucial, as proportions alone do not always determine quality.

The Impact of Poor Cut Quality

A poorly cut diamond can appear dull and lifeless, even if it has excellent clarity and color.

Common Cut Quality Issues:

• Too Deep: Light leaks through the bottom, reducing brilliance.
• Too Shallow: Light escapes through the sides instead of reflecting.
• Bow-Tie Effect: Common in ovals and marquise diamonds, where a dark area appears in the center due to improper facet alignment.

Choosing a diamond with ideal proportions and light performance verification ensures maximum brilliance and sparkle.

Should You Pay More for an Ideal Cut?

A well-cut diamond is always worth the investment, as cut quality directly impacts a diamond’s brilliance, fire, and overall visual appeal. While it may be tempting to opt for a larger diamond with a lower cut grade, this often results in a dull, lifeless stone.

A well-cut diamond reflects light efficiently, making it appear larger and brighter than a poorly cut diamond of the same carat weight. A 1-carat Ideal Cut diamond can look more striking than a 1.2-carat diamond with a lower cut grade due to its superior light return.

Studies in light physics and diamond optics show that well-cut diamonds have a higher total internal reflection, meaning more light is directed back to the viewer’s eye. Poorly cut diamonds, especially those with excessive depth or a shallow pavilion, allow light to escape through the bottom or sides, reducing brilliance.

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As with all aspects of the 4 C’s, it is worth investing in a diamond that matches your budget as well as your desires but one thing we do not suggest is compromising on the Cut. A smaller carat diamond can be made to look bigger with an excellent cut, just as one which is has a faint yellow hue can look more white with a good cut. Cut is king.