Proportions · Chapter 13
Reading proportion combinations
How crown, pavilion, table, depth, minor facets, and girdle work as one optical system.
Proportion numbers become useful only when read in combination. The same crown angle can support a very different result when the pavilion, table, lower halves, girdle, and facet precision change around it.
Crown and pavilion share the optical work
The crown helps admit light and creates opportunities for dispersion; the pavilion redirects much of that light toward the viewer. A steeper crown may pair successfully with one pavilion range and poorly with another. Neither angle carries a verdict in isolation.
Table size changes the balance
A larger table opens a broader central window into the stone and generally reduces crown height when other dimensions are held constant. A smaller table can support more crown area and fire, but only within a coherent design. Table percentage is a trade-off, not a ranking.
Depth and diameter reveal weight distribution
Total depth is read against face-up measurements and girdle thickness. Excess depth, a thick girdle, or both can place carat weight where it contributes little to visible spread. A shallow stone may spread broadly yet sacrifice pattern or durability, depending on the full combination.
Lower halves and stars tune the pattern
Longer lower-half facets tend to create thinner pavilion-main reflections and a finer, more divided pattern; shorter lower halves create broader reflections. Star-facet length affects the crown pattern and interacts with table size and upper-girdle facets. These are aesthetic and optical variables, not independent quality ladders.
The girdle is not one uniform number
A report summarizes girdle thickness as a range, while the physical edge varies around hills and valleys. Very thin areas can raise durability concerns; substantial thickness can retain hidden weight. Painting or digging changes girdle-related facet relationships and may alter face-up pattern.
Ranges are filters, not proof
Published “ideal” ranges can remove obvious outliers, but they cannot reproduce three-dimensional facet data, rounding, local variation, transparency, or a moving human view. Use combined proportions to form a hypothesis, then test it with consistent imagery and observation.
Worth remembering: A promising proportion set earns a closer look; it does not replace one.
Essential language
- Crown angle
- The angle of the bezel facets above the girdle plane in a round brilliant.
- Pavilion angle
- The angle of the pavilion main facets below the girdle plane.
- Lower-half length
- The reported relative length of lower-girdle facets, which helps shape the pavilion reflection pattern.
- Star length
- The relative length of star facets between the table and upper-girdle facets.
- Weight ratio
- The relationship between a diamond’s weight and its expected face-up diameter.
Questions to take with you
- How do the crown and pavilion angles work together rather than separately?
- Does total depth correspond to competitive face-up measurements?
- What pattern character do the lower-half and star lengths suggest?
- Does girdle variation introduce hidden weight or a vulnerable area?
- Are report values rounded averages that need exact-stone confirmation?