Scintillation

Scintillation is the pattern of bright and dark flashes a diamond produces when it, the viewer, or the light source moves. It is the third optical effect (alongside brilliance and fire) that defines what a buyer perceives as "sparkle" in a faceted diamond. Brilliance is the static total light return; fire is the spectral colour; scintillation is the dynamic flash pattern as the geometry shifts.

Scintillation is produced by the alternating bright/dark facet contrast pattern. Each facet either returns light to the eye (bright) or reflects light from elsewhere — usually the observer's own head and shoulders, which produce darkness (dark). As the stone moves through different angles, individual facets shift between bright and dark, producing the flash pattern the eye reads as sparkle.

The scintillation pattern is a function of facet count, facet size distribution, and facet symmetry. A round brilliant with 57 facets produces a finer scintillation pattern than a step cut with 25 facets. Higher facet counts (chevron-modified princess, modified cushion brilliants) produce finer "crushed-ice" scintillation; lower facet counts produce broader flashes. AGS Light Performance grading reads the scintillation pattern directly via ray-trace and grades it as part of the contrast metric.

The size of individual flashes matters. Large, balanced contrast flashes — what AGS Ideal-0 rounds produce — read as "scintillating" or "lively." Tiny, evenly-distributed flashes read as "crushed ice" or "frosted." Washed-out scintillation with weak contrast reads as "flat." The trade prefers large balanced flashes on rounds because the eye reads them as the most "diamond-like" pattern.

Scintillation interacts with light source. Direct point sources (jewellery-counter spotlights, daylight from windows) produce sharper scintillation with stronger contrast. Diffuse light sources (cloudy daylight, indirect indoor lighting) produce softer scintillation with more even illumination. The scintillation a stone shows in a jewellery store under spotlight lighting is the maximum the stone produces; the scintillation it shows in everyday wear is usually softer.

Crushed ice is the scintillation pattern characteristic of radiant cuts and modified cushions with high chevron counts. Crushed ice produces fine, even scintillation with low contrast and no broad flashes — some buyers prefer the frosted glow, others read it as "no broad flash" and prefer a step-cut or fewer-chevron cushion. Neither preference is wrong; both are visual aesthetics the cutter has dialed in through chevron count and facet geometry.