RI 1.55, SG 2.63, doubly refractive — three stones fit. Here's how to name the right one.

Why three stones fit the same readings

The refractive index window around 1.54–1.57 is one of the most crowded neighborhoods in gemology. Take a reading near RI 1.55 on a transparent, doubly refractive faceted stone and — before you run another test — your candidate list includes all of these:

All values are from the GemID reference database. Notice the honest problem: the SG ranges overlap too. For this crowd, a hydrostatic reading of 2.63 eliminates almost nothing. RI and SG get you to a shortlist — the polariscope, the dichroscope, ultraviolet light, and the loupe get you to a name. That is the skill every identification exam actually tests, and it's what the three walkthroughs below practice.

(The same RI window also holds aggregates like the chalcedony family at 1.53–1.543 — but a polariscope showing true double refraction removes them before this exercise even starts. Labradorite and rainbow moonstone begin at RI 1.559, so a clean reading at 1.55 leaves them out as well.)

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.

The pattern behind all three answers

Three stones, one method: measure, list every candidate the numbers admit, then eliminate with the test that actually discriminates. RI narrows the field. SG confirms (and in crowded neighborhoods, honestly refuses to decide). The optic figure splits uniaxial from biaxial. The dichroscope, the UV lamp, and the loupe finish the job. On exam day nobody asks how fast you got a reading — they ask whether you eliminated every wrong answer before you wrote the right one.

This elimination workflow is the same one GemID's identification engine runs across 130 species, and the same one its practice exams score you against.

Get the full practice exam

The three stones above are the free sample. The full practice exam works through a complete exam-length stone set the same way — readings first, elimination path, deciding test, answer — for the stones that show up on the GIA, Gem-A FGA, and FEEG syllabi.

Free downloads, no email needed: GIA flashcard deck · FGA/FEEG deck · confusion-pairs deck — import into Anki or Quizlet.

Study guides for each exam