Reference: Glossary

Quick definitions of terms used in GemDiagram and faceting design.

A

Angle: The tilt of a facet in degrees (0–90°), measured from the horizontal girdle plane. Steep angles (e.g., 80°) are near-vertical; shallow angles (e.g., 20°) are nearly flat.

Auto-distance: A checkbox in the tier editor that automatically recalculates a tier's distance when you change its angle, keeping the facet anchored to the girdle edge.

C

Cone angle: The half-angle of the imaginary cone a tier forms (calculated from angle and distance). Useful for understanding facet proportions.

Critical angle: The minimum angle at which light can refract out of a gemstone without undergoing total internal reflection. Calculated as arcsin(1 / refractive index). Pavilion angles must exceed critical angle to avoid windowing.

Culet: The tiny facet at the very bottom (or apex) of a pavilion. On brilliant cuts, it's a small octagon or rarely left off entirely (pointed culet).

Custom RI: A refractive index you type in yourself (1.2–3.5) instead of picking a built-in material, chosen via the Custom option in the Material dropdown. Sets the RI used by the renderer, stat tiles, and report; the other optical properties fall back to a nearby built-in material.

Cut distance: See distance.

D

Depth: The distance from the table (top) to the culet (bottom) of the finished stone.

Dispersion: The splitting of white light into its spectrum colors (fire) due to the material's wavelength-dependent refractive index. Diamonds show subtle dispersion; cubic zirconia shows intense rainbow flashes.

Distance: How far a facet plane sits from the center of the stone (perpendicular to the plane itself). Controls facet size. Usually expressed as a fraction or percentage of the girdle radius.

Dop: The cutting stick on a lapidary machine — the stone is mounted on it, and the dop's angle is adjusted to cut each facet.

F

Facet: A single flat face on the cut stone. A Round Brilliant has 57 facets; more complex cuts have more.

Fold count: The number of times a design repeats around the stone (4-fold = 4 repeats, 8-fold = 8 repeats, etc.).

Frosting: A light surface roughness applied to facet edges, creating a matte appearance instead of sharp reflections.

G

Girdle: The thin band at the widest edge of the stone, where the crown and pavilion meet. Defines the outline diameter.

Glossary: That's this. A list of terms.

I

Index wheel (or 96-index wheel): A dial with 96 equally-spaced positions around a circle. Tiers pick their positions from this wheel — e.g., "Pavilion mains at positions 0, 12, 24, 36, 48, 60, 72, 84" means 8 facets evenly spaced.

Index position: A specific position on the 96-index wheel (0–95). Each tier lists which positions it occupies.

L

Light return (or brightness): How much light entering the stone exits back to the eye. High light return = bright, sparkly stone; low light return = dark, dull stone.

M

Material: The gemstone type (Diamond, Ruby, Quartz, etc.). Each material has optical properties (refractive index, dispersion, color, density) that affect how light behaves in the cut.

Mirror (symmetry): Whether the design is symmetric across a plane through the stone. Most real cuts use mirror symmetry.

P

Pavilion: The bottom half of the cut, below the girdle. Bounces light back up into the crown.

Pleochroism: The property of showing different colors when viewed from different directions. Alexandrite (Chrysoberyl) is the only common pleochroic gemstone; GemDiagram models it with a per-axis color triplet.

Proportion ratio (or proportion): A dimensionless number describing the shape, e.g., depth/width ratio, table/width ratio. Used to compare cuts without reference to absolute size.

R

Refractive index (RI): How much a material bends light. Higher RI = more bending = brighter sparkle (for a well-cut stone). Diamond (RI 2.417) bends light more than Quartz (RI 1.544).

S

Specific gravity (SG): The density of a material relative to water. Used to convert stone volume into weight (carats). Higher SG = heavier for the same size.

Symmetry: The pattern of repetition around the stone. Defined by fold count (how many repeats) and mirror (whether each repeat is mirrored). E.g., "4-fold + mirror" = Round Brilliant symmetry.

T

Table: The flat facet at the very top of the crown. Usually a small octagon on brilliant cuts; size is adjustable in the Developer.

Tier: A ring of facets that share the same angle and distance. A Round Brilliant has 6 tiers (P1, P2, C1, C2, C3, Girdle).

Tier color: A flat color assigned to each tier in the viewport for visual identification (e.g., P1 is red, P2 is orange, etc.).

Total internal reflection (TIR): When light tries to exit a denser medium (like a gemstone) at too shallow an angle, it bounces back instead. Critical for pavilion angle — if too shallow, light leaks out the bottom (windowing) instead of bouncing back.

W

Windowing: A defect where light escapes through the pavilion instead of bouncing back up. Occurs when pavilion angles are shallower than the critical angle for the material. The stone looks transparent or dull at certain angles.

Y

Yield: The percentage of rough gemstone that survives as the finished cut. E.g., 40% yield means 40% of the rough becomes polished stone; 60% is lost to cutting, polishing, and breakage.

For more detailed explanations of faceting concepts, see Faceting Workflow, Refractive Index & Dispersion, and The 96-Index Wheel.