The newspaper test: place the diamond face-down on newspaper text. If you can read the text through the stone, the cut is too deep or the light return is poor.
I did this casually four months after buying. I could read the text. Clearly.
My stone: GIA 1ct G-VS2, cut grade: Excellent. On paper, this is a well-cut stone.
Here is the problem: GIA Excellent covers a range. The acceptable Excellent range for depth is 59–62.3%. My stone was 62.1% — technically Excellent, physically at the deep end of the range where light leakage through the pavilion begins.
What the certificate says vs what matters:
GIA grades: Excellent, Very Good, Good, Fair, Poor.
What the certificate does NOT tell you:
• Table %: should be 53–58% for maximum fire
• Crown angle: should be 34–35° for optimal light return
• Pavilion angle: should be 40.6–41° — this is the single most important number
• Crown height: 15–17% is ideal
My stone: pavilion angle 41.6°. That is the cause of the newspaper test result. Light that should reflect back through the table is leaking out the bottom.
What I do differently now:
• I filter by pavilion angle 40.6–41.0° before looking at anything else
• I use Holloway Cut Adviser (free online tool) to score any stone before buying
• GIA Excellent is a minimum bar, not a guarantee of performance
If you are shopping right now: request the full proportion data beyond what the basic certificate shows. A 0.4° pavilion angle difference is invisible on paper and visible in your ring.
Pavilion angle is the single most important number on a round brilliant certificate and it is systematically underemphasized. 40.6–41.0° is the accepted ideal range. Every degree outside that range costs you light return. GIA Excellent extends to 40.4° on the shallow end and 41.8° on the deep end — both produce noticeably different optical performance despite carrying the same grade.