GIA G230: Cut Proportions & Weight Estimation Cheat Sheet

GIA G230: Cut Proportions & Weight Estimation

Visual estimation benchmarks, weight formulas, and GIA cut grade parameters.

1. Visual Estimators

Estimating proportions by eye is a core skill. Use these visual benchmarks without a measuring device.

Table Percentage (Face-Up)

"Star Facet Bowing" method (where star facets meet kite facets):

  • →
    $60\%$ Table (Standard): Lines appear perfectly straight, creating a square outline.
  • →
    Less than $60\%$ ($54\%-57\%$): Star facets bow outward toward the girdle.
  • →
    Greater than $60\%$ ($64\%-68\%$): Star facets bow inward toward the culet.

Crown Angle (Profile)

  • →
    $34.5^\circ$ (Ideal): Slightly > 1/3 of a $90^\circ$ angle.
  • →
    $30^\circ$ (Shallow): Exactly 1/3 of a $90^\circ$ right angle.
  • →
    $45^\circ$ (Steep): Exactly 1/2 of a $90^\circ$ angle (looks noticeably tall).

Pavilion Depth (Face-Up)

  • →
    Excellent ($\sim 43\%$ Depth / $41^\circ$ Angle): Bright, even light return. Culet is a tiny dot.
  • →
    Too Shallow ("Fisheye"): Angle $<39^\circ$. Girdle reflects under table (ugly gray ring).
  • →
    Too Deep ("Nailhead"): Angle $>48^\circ$. Light leaks out bottom (center turns black).

2. Weight Estimation

For mounted diamonds measured in millimeters ($L$, $W$, $D$). Add WCF ($+1\%$ to $+10\%$) for extremely thick girdles/large culets.

Round Brilliant Cuts

$$Avg\ Dia = \frac{L + W}{2}$$
$$Carat = Avg\ Dia^2 \times Depth \times 0.0061$$

General Fancy Formula

$$Carat = L \times W \times D \times Adj\ Factor$$
  • Oval Brilliant:
    $$Carat = Avg\ Dia^2 \times Depth \times 0.0062$$
  • Heart Shape: Uses 0.0059
  • Trillion / Triangular: Uses 0.0057

Square & Rectangular L:W Adjustments

(Emerald, Radiant, Princess)

1.25:1 → 0.0080
1.50:1 → 0.0090
2.00:1 → 0.0100
2.50:1 → 0.0105

3. GIA Cut Grades

Standard Round Brilliant General Boundaries.

Excellent (EX)

Top Tier

Max brightness, fire, scintillation. Even pattern.

Table: 52% - 62%
Total D: 57.5% - 63%
Cr Angle: 31.5° - 36.5°
Pav Angle: 40.6° - 41.8°

Very Good (VG)

High light return, minor deviations. Great value.

Slightly wider params (e.g., Table up to 64%, slightly steeper cr/pav).

Good (G)

Acceptable return, somewhat dark or lacking fire.

Steeper crowns, deeper pavilions, or wide tables (>65%).

Fair (F)

Noticeable lack of brilliance. Distinctly dull/dark.

Very shallow (minor fisheye) or very deep (early nailhead).

Poor (P)

Prominent dark areas, severe light leakage.

Extreme proportions. Full nailhead or fisheye effects obvious.

All Study Guides