Gemology Fast Track / Unit 9 of 24 · Identification · 60 min
The ID workflow and the RI map
Goal: Run a repeatable test sequence and narrow 310 gems to 1–3 candidates fast.
1 · LookColour, lustre, phenomena, cut style, wear on facet edges, doubling.
2 · LoupeInclusions, bubbles, striae, join lines (doublets), surface.
3 · RI + bireNarrows to a short list. Optic character + sign.
4 · PolariscopeConfirms SR / DR / AGG / ADR.
5 · SGSeparates RI look-alikes (topaz vs tourmaline).
6 · ConfirmDichroscope, CF, UV, spectrum, inclusions typical for the species.
RI bands: who lives where
1.43–1.50Fluorite 1.43 · Opal 1.37–1.47 · Sodalite 1.48 · Obsidian/moldavite 1.48–1.51
1.51–1.56Feldspars · Quartz 1.544–1.553 · Chalcedony · Iolite · Scapolite · Amber 1.54
1.56–1.60Beryl 1.56–1.60 · Labradorite · Howlite
1.60–1.65Topaz 1.61–1.64 · Tourmaline 1.62–1.64 · Nephrite 1.61 · Turquoise 1.61 · Apatite · Andalusite · Danburite
1.65–1.70Peridot 1.654–1.690 · Spodumene 1.66–1.68 · Jadeite 1.66 · Diopside · Enstatite · Kornerupine · Tanzanite 1.69–1.70
1.70–1.76Spinel 1.712–1.720 (synth. 1.728) · Pyrope · Grossular · Tsavorite · Kyanite · Rhodolite · Chrysoberyl 1.746–1.755 · Rhodonite
1.76–1.81Corundum 1.762–1.770 · Almandine · Spessartine · Benitoite · Painite
OTL >1.81Demantoid · Zircon · Sphene · CZ · Diamond 2.417 · Moissanite · Strontium titanate · Rutile
Classic confusions and the separating test
Ruby ↔ red spinel ↔ garnetPolariscope (spinel/garnet SR), RI (spinel 1.718, ruby 1.762–1.770), garnet no fluorescence.
Topaz ↔ tourmalineBire (topaz 0.008–0.010 B+, tourmaline 0.018+ U−), SG 3.53 vs 3.06.
Aquamarine ↔ blue topazRI 1.57–1.58 vs 1.61–1.62; SG 2.70 vs 3.53.
Tanzanite ↔ iolite ↔ sapphireRI 1.69–1.70 / 1.54–1.55 / 1.76–1.77.
Emerald ↔ tsavorite ↔ green tourmalineTsavorite SR 1.74; emerald 1.57–1.60 U−; tourmaline bire 0.018–0.020.
Diamond ↔ moissanite ↔ CZDoubling (moissanite), SG/heft (CZ 5.8), conductivity testers.
Jadeite ↔ nephrite ↔ serpentineSpot RI 1.66 / 1.61 / 1.56; SG 3.34 / 2.95 / 2.57.
Peridot ↔ sinhaliteBoth high bire; peridot B+ SG 3.34, sinhalite B− SG 3.48.
Members can open the ID Helper tool: enter what you measured and it ranks every gem in the database.
Practical
Re-sort the stones from Unit 1 using the ID Helper with every number you can measure (SG with your scale at minimum). Compare to your colour-based guesses.
Unit quiz
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