The full worked practice exam — 20 stones, every answer explained

Stone 1 — the ad's exact readings

You're handed a transparent violet-blue faceted stone. Your readings:

Step 1 — what the RI admits

At 1.550, the table above is your shortlist: iolite (1.54–1.56), scapolite (1.54–1.572), sunstone (1.537–1.572) — and quartz (1.544–1.553), which for a violet-blue stone means amethyst. Four candidates, one reading.

Step 2 — what SG does and does not do

SG 2.63 is consistent with iolite (2.57–2.66), scapolite (2.50–2.74), sunstone (2.62–2.65), and amethyst (2.63–2.65). It eliminates nothing here. Don't force it — record it as confirming data and move on. Knowing when a test is not the deciding one is exam skill.

Step 3 — the optic figure

Work the stone on the polariscope until you resolve an interference figure: it's biaxial. That single observation eliminates amethyst and scapolite, which are both uniaxial. Two candidates left: iolite and sunstone, both biaxial.

Step 4 — the dichroscope

Through the dichroscope, the stone shows strong trichroism: violet-blue, light blue, and yellow-gray depending on direction. Sunstone's pleochroism is weak — pale tones you have to hunt for. Strong three-color pleochroism in this RI neighborhood points one way. Supporting check: under both longwave and shortwave UV the stone is inert, consistent with iolite.

Answer: consistent with iolite (cordierite) — famous for pleochroism so strong it's often visible to the unaided eye.

Stone 2 — same neighborhood, different answer

You're handed a transparent violet faceted stone. Your readings:

Step 1 — what the RI admits

1.556 sits inside iolite (1.54–1.56), scapolite (1.54–1.572), and sunstone (1.537–1.572). Quartz tops out at 1.553 — so a clean, repeatable 1.556 excludes amethyst on paper. But 0.003 is within the margin of a rushed reading, so a careful candidate keeps amethyst in mind until another test confirms.

Step 2 — SG

2.62 is consistent with all three remaining candidates (and with quartz at 2.63–2.65, given normal hydrostatic scatter). Again: confirming data, not deciding data.

Step 3 — the optic figure

This time the interference figure is uniaxial. That eliminates iolite and sunstone — both biaxial. What's left is scapolite, with amethyst as the lingering maybe. Two more observations settle it. First, the stone shows moderate doubling of the back facets under the loupe, which fits scapolite. Second, if you can resolve the optic sign: scapolite is uniaxial negative, quartz uniaxial positive.

Step 4 — the UV lamp

Under longwave UV the stone fluoresces strong orange. Very few gem species in this neighborhood respond like that — it's one of scapolite's most useful separators. Fluorescence costs you thirty seconds and here it's the free confirmation.

Answer: consistent with scapolite — the violet stone that gets mistaken for amethyst until the fluorescence gives it away.

Stone 3 — when the loupe was the answer all along

You're handed a transparent orange faceted stone with a faint metallic glitter. Your readings:

Step 1 — what the RI admits

1.544 is squarely inside quartz territory (1.544–1.553) — for an orange stone, that means citrine — and still inside iolite, scapolite, and sunstone. This is the trap reading: many candidates see 1.544, think "quartz," and write citrine without finishing the workflow.

Step 2 — SG

2.64 fits citrine (2.63–2.65), sunstone (2.62–2.65), iolite (2.57–2.66), and scapolite (2.50–2.74). Once more, SG refuses to pick a winner in this neighborhood. Note it and move on.

Step 3 — the optic figure

The interference figure is biaxial — and the quick citrine call dies right there, because quartz is uniaxial. Scapolite goes with it. Iolite and sunstone remain.

Step 4 — the dichroscope, then the loupe

The dichroscope shows only weak pleochroism — pale tones, easy to miss. That's the opposite of iolite's strong three-color show, so the weight shifts to sunstone. Now the clincher you noticed at first glance: under the loupe, the glitter resolves into flat metallic platelets — hematite and goethite inclusions producing aventurescence, the spangled schiller that defines sunstone. Supporting check: weak orange fluorescence under longwave UV fits sunstone; iolite is inert.

Answer: consistent with sunstone (plagioclase feldspar) — named by the inclusions that were visible before you touched a single instrument.

Stone 4 — the red stone everyone wants to name on sight

A transparent red faceted stone. Resist naming it — work it.

Step 1 — RI with birefringence

1.766 with a doubled shadow edge admits the corundum family (1.762–1.770) and its neighbors — the reading alone leaves over twenty candidates across the database, but the small 0.008 birefringence is already narrowing the story.

Step 2 — SG 4.00

Hydrostatic SG of 4.00 sits inside corundum's 3.95–4.05 and eliminates the lighter reds — red tourmaline (rubellite) and red spinel both fall well below it. The field drops by a third.

Step 3 — optic character

The interference figure resolves uniaxial. Red spinel and the red garnets are singly refractive and were already struggling; uniaxial confirms corundum-shaped optics and removes the biaxial holdouts.

Step 4 — the honest finish

What survives is the corundum family itself. Here is the exam lesson nobody advertises: ruby versus pink sapphire is a color judgment, not an instrument reading. The instruments prove corundum; the trade name follows the depth of red. Call the saturated red stone ruby and say why.

Answer: consistent with ruby (corundum) — the instruments prove the species; the variety name is a disciplined color call.

Stone 5 — the blue counterpart

A transparent blue faceted stone with readings suspiciously like Stone 4's.

Step 1 — the same constants

RI 1.762–1.770, SG 3.95–4.03, uniaxial negative: this is corundum arithmetic again. The candidates that matter now are within the species — blue sapphire against its own color-change siblings — plus the blue stones that mimic it.

Step 2 — what the dichroscope adds

Strong dichroism — violetish blue against greenish blue in the two windows. Blue spinel shows nothing (singly refractive, no pleochroism); tanzanite shows three colors and reads far lower at 1.691–1.700. Two windows, two blues, RI 1.766: corundum.

Step 3 — the lighting check

Color-change sapphire exists and shares every constant. One incandescent lamp answers it: if the hue holds blue under both light sources, you are looking at blue sapphire, not its color-change sibling.

Answer: consistent with blue sapphire (corundum) — dichroic where spinel is blind and tanzanite is trichroic.

Stone 6 — green with an entourage of imposters

A transparent green faceted stone, the most faked color in the trade.

Step 1 — RI territory

1.583 lands in beryl country (1.565–1.602). Green tourmaline reads higher (1.624–1.644); peridot much higher with visible doubling; green glass is singly refractive. Fourteen candidates at the RI step alone.

Step 2 — SG confirms beryl

2.72 sits mid-range for beryl (2.67–2.78) and clears out the heavier greens. Ten candidates remain, most of them beryl's own family.

Step 3 — uniaxial, birefringence 0.006

The low birefringence with a uniaxial figure is beryl's signature. What remains after this step is six candidates, dominated by green beryl varieties.

Step 4 — chromium or not

Under the Chelsea filter the stone shows a reddish reaction — chromium coloration is consistent with emerald rather than vanadium-poor green beryl. The filter is confirmatory only; with beryl already proven, it is the right kind of last word.

Answer: consistent with emerald (chromium beryl) — beryl by the numbers, emerald by the chromium.

Stone 7 — the pale blue twins problem

A transparent pale blue faceted stone — and the classic trap of naming it on color.

Step 1 — the trap it avoids

Blue topaz is the twin everyone fears, and RI dispatches it immediately: topaz reads 1.609–1.643 and sinks at SG 3.49–3.57. At 1.580 and SG 2.72 the stone never was topaz. Eighteen candidates at the RI step become fourteen with SG.

Step 2 — beryl again, different door

1.577–1.583 with SG 2.68–2.78 is aquamarine's exact address; the uniaxial figure narrows the field to seven, essentially the beryl family.

Step 3 — inert under UV, weak pleochroism

Longwave UV: inert — consistent with iron-colored beryl (chromium and rare-earth colors tend to answer the lamp). The dichroscope shows only a weak blue-to-colorless shift, unlike the hard two-tone of sapphire.

Answer: consistent with aquamarine (beryl) — the pale blue that reads 1.58, not 1.62.

Stone 8 — purple, and honest about it

A transparent purple faceted stone.

Step 1 — the low-RI neighborhood

1.548 is crowded: twenty-seven candidates, including the iolite/scapolite corner from Stones 1–3. But the readings are already speaking — quartz runs 1.544–1.553 with SG pinned tightly at 2.63–2.65.

Step 2 — the bull's-eye

Quartz's optic figure is the exam's one freebie: a bull's-eye pattern no other common gem gives. With it, the field collapses to the quartz family — seven candidates, all quartz varieties.

Step 3 — inert UV, purple body

Longwave UV inert, consistent with quartz. Within the family, the variety name follows color exactly as with corundum: purple quartz is amethyst.

Answer: consistent with amethyst (quartz) — the bull's-eye did the heavy lifting.

Stone 9 — the heavy blue that isn’t sapphire

A transparent blue faceted stone with real heft for its size.

Step 1 — RI opens a crowded shelf

1.626 admits twenty-two candidates — tourmaline territory (1.624–1.644), topaz (1.609–1.643), danburite and friends. This is exactly the overlap zone where a single reading cannot finish the job.

Step 2 — SG is the deciding test

Hydrostatic SG: 3.53. That single number is a massacre — tourmaline stops at 3.26, danburite lower still. Topaz (3.49–3.57) is nearly alone at this density with this RI: the twenty-two candidates fall to two.

Step 3 — biaxial confirms

The interference figure is biaxial, consistent with topaz and eliminating the last uniaxial holdout. One candidate remains.

Answer: consistent with blue topaz — identified by density, the test candidates most often skip.

Stone 10 — pink with a strong personality

A transparent pink faceted stone that darkens noticeably down one axis.

Step 1 — the birefringence tell

1.634 shares its shelf with topaz and danburite — but the 0.020 birefringence is the tell. Topaz carries 0.008; a shadow-edge separation of 0.020 at this RI is tourmaline announcing itself (1.624–1.644, birefringence 0.020).

Step 2 — SG seconds it

3.06 sits inside tourmaline's 3.01–3.26 and outside topaz's 3.49–3.57. Eleven candidates after SG, and the heavyweights are gone.

Step 3 — uniaxial + hard dichroism

Uniaxial figure, and the dichroscope shows the strong two-tone pink that tourmaline is famous for — the crystal is visibly darker down its length. Seven candidates remain, all tourmaline varieties; pink tourmaline is rubellite by trade name.

Answer: consistent with rubellite (pink tourmaline) — birefringence flagged it before SG confirmed it.

Stone 11 — the green that doubles everything

A transparent yellow-green faceted stone. Through the loupe, the back facets look printed twice.

Step 1 — RI and the doubled facets

1.672 admits seventeen candidates, but visible doubling through the table narrows attention immediately: 0.036 birefringence is enough to double back-facet edges to the naked loupe. Very few stones in this RI range can do that.

Step 2 — SG and the biaxial figure

3.34 (peridot: 3.27–3.48) with a biaxial figure brings the field to five. The doubling observation then eliminates the low-birefringence survivors — what remains are the strong doublers: peridot and two collector stones, chrome diopside and clinohumite.

Step 3 — color and habit close it

Chrome diopside is a deeper bottle-green; clinohumite runs yellow-brown. Oily yellow-green with this doubling and these constants is peridot's whole identity — the stone with essentially one color and no treatments to argue about.

Answer: consistent with peridot (olivine) — the doubling was visible before the refractometer confirmed why.

Stone 12 — three colors in one stone

A transparent yellow-green faceted stone with something odd in the dichroscope.

Step 1 — high RI, biaxial

1.750 with a biaxial figure is a short list: chrysoberyl reads 1.746–1.755 and, unusually for this RI neighborhood, is biaxial where corundum is uniaxial. Fifteen candidates at RI become four after SG (3.70–3.78) and the figure.

Step 2 — trichroism

Rotating the stone through the dichroscope produces THREE colors — possible only in biaxial stones, and chrysoberyl's documented signature. The corundums, uniaxial, cannot do this at any orientation.

Step 3 — inert UV

Longwave UV: inert, consistent with iron-colored chrysoberyl. Three candidates remain — chrysoberyl, its cat's-eye variety (which this faceted transparent stone visibly is not), and a data-sheet stray eliminated on transparency alone.

Answer: consistent with chrysoberyl — biaxial and trichroic where everything else at RI 1.75 is neither.

Stone 13 — blue-violet with three answers in the windows

A transparent blue-violet faceted stone of suspicious beauty.

Step 1 — a tight RI address

1.691–1.700 is a narrow band and the reading of 1.696 lands twelve candidates, quickly cut to nine by SG 3.35–3.38. Sapphire, the color rival, reads 1.762–1.770 and was never in the room.

Step 2 — trichroism again

Three colors in the dichroscope — blue, purple, brownish-red by orientation. That biaxial-only signature reduces the field to two: tanzanite and diaspore.

Step 3 — the lamp decides

Diaspore is a color-change stone — it shifts toward pinkish tones under incandescent light. This stone holds its blue-violet under both sources. Weak reddish-brown longwave fluorescence is likewise consistent with tanzanite.

Answer: consistent with tanzanite (zoisite) — trichroic, and steady where diaspore changes its mind.

Stone 14 — red, silent in the polariscope

A transparent red faceted stone that refuses to blink.

Step 1 — SR at 1.724

A single clean shadow edge at 1.724 with singly refractive optics: red spinel's exact profile (1.712–1.736, SG 3.57–3.63). Ruby fails twice — wrong RI and doubly refractive. Sixteen candidates at RI; SG trims to thirteen; SR optics to eight.

Step 2 — the magnet earns its keep

The garnets are the remaining threat, and most of them drift on a strong neodymium magnet. This stone shows no response — consistent with spinel, inconsistent with the iron-manganese garnets. Five candidates remain, and the red ones among them thin to spinel itself.

Step 3 — the honest caveat

Natural versus synthetic spinel shares these constants; strain patterns and inclusions under magnification are the separator, and certainty belongs to the laboratory. The species call, though, is closed.

Answer: consistent with red spinel — singly refractive where ruby cannot be, unmagnetic where garnet rarely is.

Stone 15 — the garnet that grabs the magnet

A transparent brownish-red faceted stone with unexpected weight.

Step 1 — nearly off the scale

1.800 reads high on the refractometer, close to the 1.81 ceiling — almandine's documented range is 1.77–1.83, so a stone at its top end would simply read Over The Limit. Only seven candidates live up here at all.

Step 2 — heavy and singly refractive

SG 4.05 (almandine: 3.93–4.30) and SR optics — with the tell-tale weak anomalous strain blinking that garnets are known for — cut the field to four.

Step 3 — the drag test

Strong, unmistakable magnet response. Iron-aluminum garnet chemistry is the most magnetic thing in the ordinary gem case, and the survivors after this observation are almandine and its star variety — settled by the absence of asterism in this faceted, transparent stone.

Answer: consistent with almandine garnet — high RI, dense, singly refractive, and hungry for the magnet.

Stone 16 — over the limit, doubled to the eye

A transparent blue faceted stone with fire beyond its color grade — and back facets doubled like railway tracks.

Step 1 — OTL is information

No shadow edge appears: the stone reads beyond the refractometer's ~1.81 ceiling. Record Over The Limit and treat it as data — only twenty-two candidates in the database can do this, and most are famous.

Step 2 — density sorts the OTL club

SG 4.69 (zircon: 4.60–4.80) removes nearly everyone: diamond floats away at 3.52, cubic zirconia sinks past 5.6. Two candidates survive the scale.

Step 3 — the doubling seals it

Zircon's 0.059 birefringence doubles back facets so hard it is visible through the table at a glance — the classic sleepy look. A uniaxial figure completes a profile no other blue stone matches.

Answer: consistent with blue zircon — over the limit, heavy, and doubled to the naked eye.

Stone 17 — the colorless stone with the reputation

A colorless faceted round with exceptional fire, loose on the pad.

Step 1 — OTL, colorless edition

Over the limit and colorless: the shortlist is diamond and its simulants. SG is the quiet killer here — 3.52 (diamond: 3.50–3.53) excludes cubic zirconia (5.6–6.0) and moissanite (3.20–3.22) in one weighing. Eight candidates fall to five, all of them diamond's own color varieties.

Step 2 — SR optics

Singly refractive, no doubling anywhere in the stone. Moissanite — the one simulant a thermal pen also calls “Diamond” — is doubly refractive at 0.043 and doubles its facets visibly. This stone doesn't.

Step 3 — the thermal pen, in its place

The tester reads “Diamond,” which is consistent — but note the order: SG and optics had already finished the identification. The pen confirms; it never decides alone, because moissanite defeats it.

Answer: consistent with diamond — proven by density and optics, merely agreed to by the thermal pen.

Stone 18 — the pen’s blind spot, exposed

Another colorless brilliant, allegedly the same as Stone 17.

Step 1 — the pen lies by design

The thermal tester says “Diamond” — and that is exactly why this stone is in the set. Silicon carbide conducts heat like diamond does. If your identification ended at the pen, it ended wrong.

Step 2 — SG breaks the tie

3.22 against diamond's 3.50–3.53: on a loose stone, one hydrostatic weighing separates what the pen cannot. The OTL shortlist collapses to a single candidate.

Step 3 — doubling confirms without instruments

Doubly refractive with 0.043 birefringence — back facets doubled through the table, which diamond (singly refractive) can never show. On a mounted stone where SG is unavailable, this loupe observation carries the whole identification.

Answer: consistent with moissanite — the simulant the thermal tester was built to miss.

Stone 19 — dense beyond reason

A colorless faceted stone with a strange, almost leaden heft.

Step 1 — the heft is the headline

Over the limit and singly refractive like diamond — but SG 5.80. Nothing in the diamond conversation weighs this much: cubic zirconia's 5.6–6.0 makes it half again denser than diamond, and in the hand a one-carat-size CZ simply feels wrong.

Step 2 — the pen agrees for once

“Simulant” on the thermal tester — CZ conducts heat poorly, and this is the reading the pen was actually designed to produce. Two candidates survive the density cut, and the other is a deep-red collector mineral this colorless stone cannot be.

Step 3 — the size-to-weight check

Carat weight roughly 1.7× what the stone's diameter promises for diamond: the oldest CZ tell, measurable with a scale and a millimeter gauge alone.

Answer: consistent with cubic zirconia — convicted by density before any electronics were involved.

Stone 20 — the one that never goes dark

A translucent green cabochon with a granular glow.

Step 1 — the polariscope speaks first

The stone never goes dark at any rotation: an aggregate reaction, the signature of polycrystalline material. That single observation removes every single-crystal candidate — seventeen stones at this RI become one family question.

Step 2 — spot RI and SG

A distance-vision spot reading near 1.67 (jadeite: 1.654–1.688) with SG 3.34 (3.24–3.43) is jadeite's documented address. Nephrite, the classic confusion, reads lower on both counts and shows a fibrous rather than granular texture under the loupe.

Step 3 — the honest boundary

Species: closed. But jadeite's real-world question — natural color versus dye versus polymer impregnation — belongs to laboratory spectroscopy. Write the species with confidence and the treatment status as “undetermined without lab testing.”

Answer: consistent with jadeite jade — an aggregate answer the polariscope gave away in the first minute.

Keep practicing

Reading worked answers builds the map; running stones against the clock builds the skill. The Loupewise app deals timed practice exams in the GIA 20-stone, Gem-A FGA, and FEEG formats, scores them, and explains every answer the way this page does — start a practice exam.

Loupewise is an independent study tool. It is not affiliated with, endorsed by, or sponsored by GIA, Gem-A, or FEEG. Property ranges are compiled from published gemological references; always verify important identifications with an accredited laboratory.