Gemological Glossary

The angle between the bezel facets and the table plane on a round brilliant — the proportion that pairs with pavilion angle to determine light return.

The angle between the pavilion main facets and the girdle plane — the single most sensitive proportion in the brilliance equation.

The width of the flat top facet as a percentage of the average girdle diameter — the proportion that trades fire for spread.

Total height of a diamond divided by average girdle diameter — the proportion that exposes weight hidden under the girdle.

The width of the narrow band where the crown meets the pavilion — GIA grades it Extremely Thin to Extremely Thick.

The tiny facet or point at the very bottom of the pavilion — GIA grades it None to Extremely Large.

The eight large kite-shaped facets on the bottom of a round brilliant that meet at the culet — the facets that drive total internal reflection.

The eight large kite-shaped facets on the crown of a round brilliant — the facets that carry most of the fire.

The eight small triangular facets between the table and bezel facets — star length tunes scintillation at the center of the stone.

A visual pattern of eight symmetric arrows face-up and eight hearts face-down seen in round brilliants cut to exacting symmetry.

The top tier on the AGS Performance Grading Standard — zeroes on Light Performance, Polish, Symmetry, and Proportion.

The total amount of white light a diamond returns to the eye — a function of crown and pavilion angles, table size, and polish.

The flashes of spectral color a diamond throws when white light splits into its component wavelengths — driven by crown angle and table size.

The pattern of bright and dark flashes a diamond produces when it, the viewer, or the light source moves.

A fine, frosted scintillation pattern characteristic of radiant cuts and modified cushions with high chevron counts.

V-shaped pavilion facets on princess, radiant, and modified cushion cuts — chevron count controls the crushed-ice vs broad-flash look.

A dark hourglass-shaped shadow across the center of elongated fancy cuts — oval, marquise, pear — caused by light leakage under the viewer's head.

How a diamond responds under longwave UV — GIA grades None, Faint, Medium, Strong, Very Strong. Help or hurt depends on body color.

The cloudy face-up appearance some Strong and Very Strong Blue fluorescent diamonds show in UV-rich light.

The 365 nm wavelength GIA uses to grade fluorescence — the same band as standard jeweller blacklights but a subset of daylight UV.

The most common diamond type — roughly 95% of natural diamond — defined by aggregated nitrogen impurities.

The rarest natural diamond type — less than 0.001% nitrogen — and the type that includes most top D-color stones and most CVD lab-grown.

The rarest natural diamond type after IIa — boron-bearing, electrically conductive, the family that produces natural fancy blue diamonds.

The dominant lab-grown method — builds diamond crystal layer by layer from a methane-hydrogen plasma at low pressure.

The older lab-grown method — grows diamond from a metal-solvent at pressures and temperatures replicating natural formation.

A lab-grown diamond that has not received any post-growth color or clarity treatment.

A high-temperature anneal applied to finished CVD lab-grown to remove brown tint and shift color toward D-F.

The dominant grading report for lab-grown diamond in US retail — IGI uses the same D-Z and FL-I3 scales as GIA.

GIA's lab-grown report — same D-Z and FL-I3 scales as natural since 2020, with growth method and treatment disclosure.

Trade name for the velvety medium-blue tone associated with Kashmir sapphire — also used as a color description by SSEF and Gübelin.

Trade name for a deeper, more vivid blue with slight violet modifier — the color most associated with top Mogok sapphire.

Pinkish-orange sapphire — the only color call in the sapphire family with its own name.

Microscopic rutile needles in corundum that scatter light and create the velvety character of fine Kashmir and Mogok sapphires.

Sapphire that has never been thermally treated to improve color or clarity — trades at multiples of heated at investment grades.

Traditional heat at 1,400-1,800°C dissolves silk, improves color and clarity — the standard treatment for over 95% of commercial sapphire.

Heat treatment with beryllium powder that penetrates the corundum lattice and creates orange or yellow color from the outside in.

Velvety cornflower-blue sapphires from the closed Padar valley deposit in northern India — the most prized source.

Sapphires from the Mogok Stone Tract in Myanmar — one step below Kashmir at the top of the market.

Heat treatment with chemical additives that penetrate the crystal lattice and create color from the outside in.

The trade color call for the finest ruby — pure vivid red with slight blue undertone, strong saturation, medium-dark tone.

A rare optical phenomenon in the finest Colombian emerald — a roiled, dappled internal pattern visible at 10× that signals slow crystal growth.

Trade term for the network of inclusions characteristic of emerald — three-phase inclusions, growth tubes, fingerprint patterns.

The lowest treatment grade SSEF, Gübelin and GRS issue on emerald — only trace amounts of oil detectable in surface-reaching fissures.

The standard commercial emerald treatment level — oil clearly present in fissures but not heavily filling.

The heaviest treatment level — oil substantially fills fractures; large discounts and maintenance risk.

A star-shaped optical phenomenon visible in cabochon-cut sapphire and ruby caused by intersecting rutile silk needles.

A single-band optical phenomenon — a thin line of light moves across the surface of cabochon-cut stones with fibrous inclusions.

The property of showing different colors when viewed from different crystallographic directions — strong in many colored stones, absent in diamond.

Two-color pleochroism — the property of showing two distinct colors along two perpendicular crystal axes. Universal in sapphire, ruby, tourmaline.

Three-color pleochroism — three distinct colors along three crystal axes. Characteristic of tanzanite, iolite, andalusite.

Laser ablation inductively coupled plasma mass spectrometry — the technique that turned origin determination from opinion into measurement.

Shortwave UV imaging instrument that separates natural from lab-grown diamond via characteristic fluorescence patterns.

Swiss Gemmological Institute origin and treatment authority for colored gemstones — auction-grade credential for top-tier lots.

Swiss gemological laboratory in Lucerne — together with SSEF, the most trusted origin authority for top colored gemstones.