Gem Faceting Lab

Drag to orbit the gem. Every slider changes real facet geometry and the Snell's-law ray path below — nothing here is decorative.

Material

Pavilion angle

Crown angle

Table size

Facet count

Light elevation

Critical angle

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Pavilion TIR

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Light return

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Fire spread

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Cross-section ray trace (Snell's law, live)

If pavilion angle drops below the critical angle for the chosen material, light leaks out the back of the stone instead of bouncing back to the eye — this is exactly why a badly-cut gem looks dull or "fish-eyed" even in flawless material.