Melbourne client care · Made in Surat
The proportions of a diamond determine how light travels through it. Get them right and the diamond is brilliant. Get them wrong and light leaks, creating a dull, lifeless stone. This guide explains every proportion and why each one matters.
A diamond's proportions are the precise geometric measurements of the cut — how wide the table facet is relative to the diameter, how deep the stone is, at what angles the crown and pavilion facets are cut. These proportions determine the path of light through the stone.
When proportions are within optimal ranges, light enters the crown, reflects internally off the pavilion facets and returns through the crown — creating maximum brilliance and fire. When proportions are outside optimal ranges, light leaks through the sides or bottom rather than returning through the crown.
This is why cut quality is the most important of the 4Cs. It determines how the diamond performs optically. A diamond with poor proportions looks poor regardless of colour or clarity. An Excellent cut diamond with modest colour or clarity looks beautiful.
| Proportion | What It Measures | Ideal Range | Effect If Wrong |
|---|---|---|---|
| Table % | Width of the flat top facet as % of diameter | 54–62% | Too large: reduced fire. Too small: dark centre appearance |
| Depth % | Total stone depth as % of girdle diameter | 59–62.5% | Too deep: light leaks from sides. Too shallow: light leaks through bottom |
| Crown angle | Angle of crown facets to girdle plane | 32–36° | Affects fire significantly. Outside range reduces fire or brilliance |
| Pavilion angle | Angle of pavilion facets — the most critical proportion | 40.6–41.8° | Even 1° outside causes significant light leakage. Most important single proportion |
| Girdle thickness | Thickness of the stone edge | Thin to Slightly Thick | Very thin: chipping risk. Very thick: weight hidden, stone appears smaller |
| Culet size | Size of the bottom point or facet | None to Very Small | Large culet: visible dark circle through the table |
Of all the proportions, the pavilion angle has the most dramatic effect on light performance. The pavilion facets are the angled surfaces below the girdle — they act as mirrors, reflecting light back through the crown. At 40.6–41.8°, light hits the pavilion at the right angle for total internal reflection, bounces and exits through the crown as brilliance.
Even a 1° variation from this range causes meaningful light leakage. A pavilion angle of 40.0° vs 41.0° creates no visible geometric difference in the stone but produces a significant visible difference in light return. This is why IGI proportion data on the certificate matters — and why Yash Chandpara reviews it for every stone.
Book a free online consultation. Yash Chandpara explains the proportions of every stone shown and why each was selected for light performance.
