Diamond Proportions Explained: The 2026 Guide to Table %, Depth % and the Numbers Behind a Diamond's Sparkle

"Same one carat, same color, same clarity—so why does this diamond sparkle so much more than that one?" The answer is often hidden in a small corner of the grading report: the proportions. These figures are what determine the "Cut" grade from the inside out. In this guide, we explain table, depth, and the other numbers in plain language for first-time buyers.

What Are Diamond Proportions?

Proportions refer to the balance of sizes and angles between a diamond's parts: the size of the flat top facet (the table), the slope of the upper half (the crown angle), and the depth and angle of the lower half (the pavilion). When these interrelate in ideal ratios, light reflects efficiently inside the stone and returns to the eye as maximum brilliance. When the balance is off, light leaks out the bottom or sides—leaving a dull stone even at the same 4C grade.

The Two Numbers to Check: Table % and Depth %

On a GIA or CGL report, you will find percentage figures alongside a profile diagram of the stone. Two matter most.

Table %

This is the size of the flat central facet, viewed from directly above, expressed as a percentage of the stone's diameter. Too large and light passes straight through; too small and the sparkle lacks spread. A flattering range is generally 52–62%.

Depth %

Depth is the height from the flat table down to the pointed tip (the culet), expressed as a percentage of the girdle diameter. Depth strongly affects a stone's brightness—too deep or too shallow and light escapes. The ideal sits around 57.5–63%.

The "Golden Ratio" Benchmarks for Brilliance

Research-derived ideal proportions are roughly: table size 52–62%, total depth 57.5–63%, crown angle 31.5–36.5°, and pavilion angle 40.6–41.8°. The pavilion angle is especially delicate—a difference of just one degree changes the path of reflected light—which is where a cutter's skill truly shows. This is precisely why ADAMAS obsesses over every single D-color diamond: realizing the finest color to its fullest demands this kind of meticulous proportion design.

Why It Matters Most for the Round Brilliant

Proportion figures are scrutinized most rigorously for the round brilliant cut. Its 58 facets, placed at precisely calculated angles, bounce light repeatedly inside the stone to create that unmatched sparkle. It is the only shape GIA formally assigns a cut grade (Excellent to Poor)—a sign of how directly the numbers translate to beauty. If you are choosing a solitaire or eternity ring where a single stone takes center stage, attention to proportions shapes your satisfaction directly.

Choosing Wisely—Beyond the Numbers

Proportion figures are a powerful guide, but final beauty is decided by overall balance. Even within the benchmark ranges, the way each element combines changes how a stone sparkles, so when possible, hold it to the light and check whether the entire table glitters evenly. A cut grade of "Excellent" or "Triple Excellent (3EX)" is reliable proof that the proportions were designed within the ideal zone. At ADAMAS, only diamonds with thoroughly calculated proportions—set in fine K18 gold—make it into our collection.

Conclusion

A diamond's brilliance is no accident; it is born from the precise design of its proportions, starting with table and depth. The next time you choose a stone, look beyond carat and color to the numbers on the report. Your search for timeless, unchanging brilliance can begin with the collection at ADAMAS (adamas-gold.jp). Find the one stone destined to shine on your hand for a lifetime.