Optic Character of Gemstones — Polariscope Reference Chart
Last updated: August 2026
Optic character for 130 gemstone species — 34 singly refractive, 74 doubly refractive, and 22 aggregate — with anomalous double refraction (ADR) flags and birefringence. Grouped SR first, then uniaxial, biaxial, and aggregate.
Common optic character lookups: Diamond SR · Spinel SR · Sapphire Uniaxial (−) · Quartz Uniaxial (+) · Emerald Uniaxial (−) · Topaz Biaxial (+) · Peridot Biaxial (+) · Jadeite Aggregate
Optic character is the first thing a polariscope tells you about a stone, and it cuts the candidate list roughly in half in under a minute. Place the stone between crossed polarizing filters and rotate it: a singly refractive (SR) stone stays dark, a doubly refractive (DR) stone blinks light and dark four times per revolution, and a polycrystalline aggregate (AGG) stays light throughout. No contact fluid, no surface requirement — it even works through the side of many mounted stones.
The finer grades — uniaxial vs. biaxial, optic sign positive or negative — come from an interference figure viewed with a conoscope lens, or from the behavior of the refractometer shadow edges. They matter because they are constants of the crystal system: quartz is uniaxial positive, tourmaline uniaxial negative, topaz biaxial positive. When two candidates share an RI range, optic sign is often the property that splits them.
The trap for beginners is anomalous double refraction. Strained singly refractive stones — garnets and diamond above all — can show patchy or banded extinction that looks like a blink. The ADR column below flags every species where the GemID engine expects this effect, so a "DR" polariscope call on a flagged stone should be confirmed with the refractometer before it eliminates SR candidates. The legend below the table explains each reaction in detail.
Studying for the GIA or FGA practical? GemID drills polariscope calls — SR, DR, AGG, and the ADR traps — and runs timed mock exams built around the real exam formats.
Practice exams →| Gem | Optic Character | ADR | Birefringence | RI Range |
|---|---|---|---|---|
| Almandine Garnet | SR (Singly Refractive) | Yes | — | 1.770–1.830 |
| Amber | SR (Singly Refractive) | Yes | — | 1.539–1.545 |
| Black Opal | SR (Singly Refractive) | — | — | 1.440–1.460 |
| Black Spinel | SR (Singly Refractive) | Yes | — | 1.712–1.730 |
| Blue Diamond | SR (Singly Refractive) | Yes | — | 2.417 |
| Brown Diamond | SR (Singly Refractive) | Yes | — | 2.417 |
| Color-Change Garnet | SR (Singly Refractive) | Yes | — | 1.740–1.765 |
| Cubic Zirconia | SR (Singly Refractive) | Yes | — | 2.150–2.180 |
| Cuprite | SR (Singly Refractive) | — | — | 2.849 |
| Demantoid Garnet | SR (Singly Refractive) | Yes | — | 1.880–1.895 |
| Diamond | SR (Singly Refractive) | Yes | — | 2.417 |
| Fire Opal | SR (Singly Refractive) | — | — | 1.370–1.470 |
| Fluorite | SR (Singly Refractive) | — | — | 1.432–1.436 |
| Grossular Garnet | SR (Singly Refractive) | Yes | — | 1.734–1.759 |
| Hackmanite | SR (Singly Refractive) | — | — | 1.483–1.487 |
| Hauyne | SR (Singly Refractive) | — | — | 1.494–1.509 |
| Hessonite Garnet | SR (Singly Refractive) | Yes | — | 1.730–1.760 |
| Jet | SR (Singly Refractive) | — | — | 1.640–1.680 |
| Low-Type Zircon | SR (Singly Refractive) | — | — | 1.780–1.815 |
| Malaya / Umbalite Garnet | SR (Singly Refractive) | Yes | — | 1.730–1.780 |
| Moldavite | SR (Singly Refractive) | — | — | 1.480–1.510 |
| Obsidian | SR (Singly Refractive) | Yes | — | 1.480–1.510 |
| Opal | SR (Singly Refractive) | — | — | 1.370–1.470 |
| Pink Diamond | SR (Singly Refractive) | Yes | — | 2.417 |
| Pyrope Garnet | SR (Singly Refractive) | Yes | — | 1.730–1.760 |
| Rhodolite Garnet | SR (Singly Refractive) | Yes | — | 1.740–1.770 |
| Spessartite Garnet | SR (Singly Refractive) | Yes | — | 1.790–1.815 |
| Sphalerite | SR (Singly Refractive) | — | — | 2.369–2.500 |
| Spinel | SR (Singly Refractive) | Yes | — | 1.712–1.736 |
| Star Garnet | SR (Singly Refractive) | Yes | — | 1.770–1.830 |
| Synthetic Spinel | SR (Singly Refractive) | Yes | — | 1.725–1.728 |
| Tsavorite Garnet | SR (Singly Refractive) | Yes | — | 1.730–1.760 |
| YAG | SR (Singly Refractive) | — | — | 1.833 |
| Yellow Diamond | SR (Singly Refractive) | Yes | — | 2.417 |
| Amethyst | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Ametrine | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Benitoite | DR Uniaxial (+) | — | 0.047 | 1.757–1.804 |
| Cassiterite | DR Uniaxial (+) | — | 0.096 | 2.010–2.100 |
| Cinnabar | DR Uniaxial (+) | — | 0.351 | 2.905–3.256 |
| Citrine | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Moissanite | DR Uniaxial (+) | — | 0.043 | 2.648–2.691 |
| Phenakite | DR Uniaxial (+) | — | 0.016 | 1.654–1.670 |
| Prasiolite | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Rock Crystal | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Rose Quartz | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Smoky Quartz | DR Uniaxial (+) | — | 0.009 | 1.544–1.553 |
| Zircon | DR Uniaxial (+) | — | 0.059 | 1.925–1.984 |
| Apatite | DR Uniaxial (−) | — | 0.006 | 1.628–1.649 |
| Aquamarine | DR Uniaxial (−) | — | 0.006 | 1.577–1.583 |
| Chrome Tourmaline | DR Uniaxial (−) | — | 0.018 | 1.624–1.660 |
| Color-Change Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Emerald | DR Uniaxial (−) | — | 0.006 | 1.565–1.602 |
| Goshenite | DR Uniaxial (−) | — | 0.006 | 1.577–1.583 |
| Green Tourmaline | DR Uniaxial (−) | — | 0.018 | 1.616–1.650 |
| Heliodor | DR Uniaxial (−) | — | 0.006 | 1.568–1.580 |
| Hematite | DR Uniaxial (−) | — | 0.280 | 2.940–3.220 |
| Indicolite | DR Uniaxial (−) | — | 0.018 | 1.624–1.644 |
| Morganite | DR Uniaxial (−) | — | 0.008 | 1.572–1.600 |
| Padparadscha Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Paraiba Tourmaline | DR Uniaxial (−) | — | 0.018 | 1.624–1.644 |
| Pink Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Red Beryl | DR Uniaxial (−) | — | 0.007 | 1.564–1.574 |
| Rhodochrosite | DR Uniaxial (−) | — | 0.220 | 1.600–1.820 |
| Rubellite | DR Uniaxial (−) | — | 0.018 | 1.624–1.644 |
| Ruby | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Scapolite | DR Uniaxial (−) | — | 0.020 | 1.540–1.572 |
| Star Ruby | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Star Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Sugilite | DR Uniaxial (−) | — | 0.002 | 1.607–1.610 |
| Synthetic Color-Change Corundum | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Taaffeite | DR Uniaxial (−) | — | 0.004 | 1.719–1.723 |
| Tourmaline | DR Uniaxial (−) | — | 0.020 | 1.624–1.644 |
| Watermelon Tourmaline | DR Uniaxial (−) | — | 0.018 | 1.616–1.650 |
| White Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Yellow Sapphire | DR Uniaxial (−) | — | 0.008 | 1.762–1.770 |
| Alexandrite | DR Biaxial (+) | — | 0.009 | 1.746–1.755 |
| Cat's Eye Chrysoberyl | DR Biaxial (+) | — | 0.009 | 1.746–1.755 |
| Chrome Diopside | DR Biaxial (+) | — | 0.030 | 1.664–1.730 |
| Chrysoberyl | DR Biaxial (+) | — | 0.009 | 1.746–1.755 |
| Clinohumite | DR Biaxial (+) | Yes | 0.028 | 1.625–1.675 |
| Diaspore | DR Biaxial (+) | — | 0.048 | 1.685–1.752 |
| Enstatite | DR Biaxial (+) | — | 0.010 | 1.663–1.673 |
| Hiddenite | DR Biaxial (+) | — | 0.016 | 1.660–1.676 |
| Kunzite | DR Biaxial (+) | — | 0.016 | 1.660–1.676 |
| Larimar | DR Biaxial (+) | — | 0.040 | 1.595–1.640 |
| Peridot | DR Biaxial (+) | — | 0.036 | 1.654–1.690 |
| Prehnite | DR Biaxial (+) | — | 0.030 | 1.611–1.665 |
| Rhodonite | DR Biaxial (+) | — | 0.014 | 1.716–1.752 |
| Sphene | DR Biaxial (+) | — | 0.100 | 1.843–2.110 |
| Star Diopside | DR Biaxial (+) | — | 0.030 | 1.664–1.721 |
| Tanzanite | DR Biaxial (+) | — | 0.009 | 1.691–1.700 |
| Thulite | DR Biaxial (+) | — | 0.009 | 1.691–1.704 |
| Topaz | DR Biaxial (+) | — | 0.008 | 1.609–1.643 |
| Amazonite | DR Biaxial (−) | — | 0.008 | 1.514–1.530 |
| Andalusite | DR Biaxial (−) | — | 0.010 | 1.629–1.649 |
| Danburite | DR Biaxial (−) | — | 0.006 | 1.627–1.636 |
| Grandidierite | DR Biaxial (−) | — | 0.033 | 1.590–1.623 |
| Iolite | DR Biaxial (−) | — | 0.012 | 1.540–1.560 |
| Kornerupine | DR Biaxial (−) | — | 0.013 | 1.660–1.699 |
| Kyanite | DR Biaxial (−) | — | 0.016 | 1.712–1.734 |
| Labradorite | DR Biaxial (−) | — | 0.011 | 1.559–1.573 |
| Malachite | DR Biaxial (−) | — | 0.254 | 1.655–1.909 |
| Moonstone | DR Biaxial (−) | — | 0.008 | 1.518–1.526 |
| Rainbow Moonstone | DR Biaxial (−) | — | 0.011 | 1.559–1.572 |
| Sinhalite | DR Biaxial (−) | — | 0.038 | 1.665–1.712 |
| Sunstone | DR Biaxial (−) | — | 0.010 | 1.537–1.572 |
| Variscite | DR Biaxial (−) | — | 0.031 | 1.560–1.590 |
| Agate | Aggregate | — | — | 1.530–1.543 |
| Aventurine | Aggregate | — | — | 1.544–1.553 |
| Bloodstone | Aggregate | — | — | 1.530–1.543 |
| Carnelian | Aggregate | — | — | 1.530–1.543 |
| Chalcedony | Aggregate | — | — | 1.530–1.543 |
| Charoite | Aggregate | — | 0.009 | 1.550–1.559 |
| Chrysocolla | Aggregate | — | — | 1.460–1.630 |
| Chrysoprase | Aggregate | — | — | 1.530–1.543 |
| Coral | Aggregate | — | — | 1.486–1.658 |
| Hawk's Eye | Aggregate | — | — | 1.544–1.553 |
| Howlite | Aggregate | — | — | 1.583–1.608 |
| Ivory | Aggregate | — | — | 1.535–1.540 |
| Jadeite | Aggregate | — | — | 1.654–1.688 |
| Lapis Lazuli | Aggregate | — | — | 1.500–1.670 |
| Nephrite | Aggregate | — | — | 1.600–1.627 |
| Onyx | Aggregate | — | — | 1.530–1.543 |
| Pearl | Aggregate | — | — | 1.520–1.690 |
| Pietersite | Aggregate | — | — | 1.534–1.540 |
| Serpentine | Aggregate | — | — | 1.560–1.571 |
| Sodalite | Aggregate | — | — | 1.483–1.487 |
| Tiger's Eye | Aggregate | — | — | 1.544–1.553 |
| Turquoise | Aggregate | — | — | 1.610–1.650 |
Click a column header to re-sort the table.
Reading the Polariscope — SR, DR, AGG, ADR
SR (singly refractive): the stone stays dark through a full 360° rotation between crossed polars. 34 species in this chart.
DR (doubly refractive): the stone blinks light and dark four times per rotation. Uniaxial or biaxial, positive or negative — the sign comes from an interference-figure or refractometer observation, not the basic blink test. 74 species.
AGG (aggregate): polycrystalline material stays uniformly light in all positions. 22 species.
ADR (anomalous double refraction): some singly refractive stones show patchy, wavy, or cross-hatched strain effects that can mimic a DR blink. The ADR column flags the 23 species where this is expected — garnets and diamond are the classic cases. An ADR reaction is irregular and directionless where a true DR blink is crisp and repeats every 90°; when in doubt, confirm with the refractometer.
Which gemstones show anomalous double refraction (ADR)?
23 species in this chart are flagged for ADR: Almandine Garnet, Amber, Black Spinel, Blue Diamond, Brown Diamond, Clinohumite, Color-Change Garnet, Cubic Zirconia, Demantoid Garnet, Diamond, Grossular Garnet, Hessonite Garnet, and 11 more. These singly refractive stones can show strain-related patchy extinction on the polariscope that mimics a doubly refractive blink — confirm with a refractometer before ruling out SR candidates.
Is diamond singly or doubly refractive?
Diamond is singly refractive (cubic crystal system), but it is flagged for ADR in this chart: internal strain very commonly produces anomalous extinction patterns on the polariscope. That combination — SR with frequent strong ADR — is itself characteristic of diamond.
What does uniaxial vs. biaxial mean?
Doubly refractive stones split into two groups by crystal system. Uniaxial stones (tetragonal, hexagonal, trigonal — e.g. quartz, corundum, tourmaline) have one optic axis; biaxial stones (orthorhombic, monoclinic, triclinic — e.g. topaz, peridot, tanzanite) have two. Each carries an optic sign, + or −, which the chart lists for every DR species.
All instrument charts: Refractive Index · Specific Gravity · Mohs Hardness · Birefringence · Fluorescence · Pleochroism · Spectroscope · Chelsea & Hanneman Filters · Dispersion · Magnetism
See also: Full Reference Database · How to Identify a Gemstone · Exam Study Guides
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