Gemology Glossary — GemID Docs

Optical Properties

Refractive Index RI: A measure of how much a gemstone bends light passing through it. Defined as the ratio of the speed of light in a vacuum to its speed in the gem. Higher RI = greater bending. Measured with a refractometer using total internal reflection. Most diagnostic single measurement in gemology — GemID narrows candidates to 5–15 with RI alone. See: RI Reference Chart, How to Use a Refractometer.
Birefringence BR: The difference between the highest and lowest RI reading of a doubly refractive gem. A high birefringence value (e.g., calcite at 0.172) causes visible facet doubling — back facet edges appear doubled when viewed through the table. Calculated as RI_max − RI_min.
Optic Character: Describes how a gem interacts with polarized light. Four main types:

SR — Singly refractive. One RI reading. Gems: diamond, garnet (most), spinel, glass.

DR U+ / DR U− — Doubly refractive, uniaxial positive or negative. One optic axis. Gems: quartz (U+), calcite (U−), corundum (U−).

DR B+ / DR B− — Doubly refractive, biaxial positive or negative. Two optic axes. Gems: topaz (B+), alexandrite (B+), tourmaline (B−).

AGG — Aggregate. Polycrystalline; shows anomalous extinction. Gems: jadeite, nephrite, chalcedony.

Determined with a polariscope.
Dispersion: How strongly a gem splits white light into spectral colors, creating "fire." Measured as the difference in RI between red (687 nm) and violet (430 nm) light. Diamond: 0.044. Demantoid garnet: 0.057 (higher = more fire).
Pleochroism / Dichroism / Trichroism: Pleochroism is the property of showing different body colors when viewed from different crystallographic directions. Doubly refractive gems can show two colors (dichroism); triaxial gems can show three (trichroism). Observed with a dichroscope. Highly diagnostic — e.g., tanzanite shows strong trichroism (blue, violet, burgundy).

Physical Properties

Specific Gravity SG: Density relative to water. A gem with SG 3.52 is 3.52× as dense as water. Measured by hydrostatic weighing (weight in air ÷ weight-loss in water). Second most diagnostic measurement after RI. See: SG Reference Chart, How to Measure SG.
Hardness (Mohs Scale): Scratch resistance on a relative 1–10 scale. Not the same as toughness (resistance to fracture). A gem at Mohs 7 will scratch one at Mohs 6. Diamond (10) is the hardest natural mineral; talc (1) is the softest. See: Hardness Chart.
Cleavage: A tendency to break along flat planes parallel to atomic bond planes in the crystal structure. Described by direction (e.g., "basal") and quality (perfect, good, poor, none). Topaz has perfect basal cleavage — relevant for cutting and handling.
Crystal System: The geometric symmetry class of the gem's crystal structure. The seven systems are: cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic. Determines optic character — cubic gems are singly refractive; all others are doubly refractive (unless amorphous).

Identification Tests

Chelsea Filter: A dichroic filter that transmits only deep red (~690 nm) and yellow-green (~570 nm) light. Gems that appear red through the Chelsea filter contain chromium (e.g., natural emerald, ruby, red spinel). Useful for distinguishing chromium-colored gems from iron-colored simulants. See: How to Use a Chelsea Filter, Fluorescence Chart.
UV Fluorescence: Luminescence (glowing) emitted by a gem under ultraviolet light. Tested at two wavelengths: longwave (LW, 365 nm) and shortwave (SW, 254 nm). Highly variable — same species can fluoresce differently depending on origin and trace elements. Useful for origin detection (e.g., Burmese rubies fluoresce strongly red LW; Thai/Cambodian rubies are weaker). See: Fluorescence Chart, UV Fluorescence Guide.
Refractometer: An instrument that measures RI using total internal reflection. A polished flat facet is placed on the hemicylinder with refractometer liquid (high-RI contact fluid). The shadow edge on the scale indicates RI. Most gemological refractometers read 1.35–1.80 (limited by the glass). For gems above the scale (e.g., diamond, zircon, sphene), only a spot reading is possible.
Polariscope: An instrument with two polarizing filters (polarizer and analyzer) used to determine optic character. When crossed (90° apart), SR gems remain dark in all positions; DR gems blink between light and dark as you rotate the stone. AGG stones show a "bull's eye" or mosaic pattern.
Dichroscope: An instrument that splits transmitted light into two polarized beams, allowing you to see a gem's two (or three) pleochroic colors simultaneously. Essential for separating tanzanite from iolite and other strongly pleochroic gems.
Spectroscope: An instrument that displays the absorption spectrum of a gem — dark bands at wavelengths where the gem absorbs light. Chromophore-specific: chromium, iron, and cobalt each produce characteristic patterns. Powerful but requires practice to read. Generally requires a desk-lamp illuminator for reflected-light use.

Results Terminology

Consistent with: A candidate gem whose documented property ranges overlap all entered measurements. Does not mean confirmed identification — it means the stone could be this gem based on the measurements taken. GemID always uses "consistent with" rather than "is." More measurements = higher confidence.
Eliminated: A gem that has been ruled out because at least one entered measurement falls outside its documented range. Tap "N eliminated" on the results screen to see which gems were eliminated and which measurement caused each elimination.
Next Test: The measurement GemID recommends you take next — the test that will most efficiently differentiate the remaining candidates. Recalculates automatically after each measurement is entered. Follow it when you have the right instrument available.
RI Tolerance: The acceptable variation applied to RI readings in GemID (±0.003 by default). Accounts for instrument calibration variation and reading error. Gems whose documented range overlaps your reading within tolerance are kept as candidates.