Study for fellowship-level identification by building separation pairs, not property lists. For every gemstone, learn which two readings taken together distinguish it from its closest neighbours, and rehearse that decision on written scenarios where colour and a single measurement would lead you astray.
Why overlapping property ranges change how you should study
Commercial gemstones sit close together in reference tables. Fellowship study trains you to select the property pair, such as refractive index with birefringence or specific gravity, that resolves each particular overlap.
In introductory study, refractive index (RI) and specific gravity (SG) are numbers to recall. At fellowship level they become a decision system. Corundum, for example, shows RI around 1.76 to 1.77 with a double refraction of about 0.008, while spinel is singly refractive near 1.718. Neither number alone is the point; the combination of a uniaxial, doubly refracting reading with a high SG separates corundum from every near neighbour in one decision step.
The working habit to build is a separation tree. Start with the stone in front of you, take one broad reading, and let it eliminate families rather than name a species. A reading near 1.77 could mean corundum, but certain garnets also approach that region, so your next reading must be one the two do not share, such as optic character. Writing these branches for each stone is more useful than re-copying full property tables.
| Ambiguity you meet | First reading | Tie-breaker reading | Why the pair works |
|---|---|---|---|
| Red stone: corundum vs spinel | Polariscope behaviour | Double refraction and SG | Spinel is singly refractive with lower SG; corundum is doubly refracting and dense |
| High-RI red stone: corundum vs almandine garnet | Optic character | UV reaction and spectrum | Both dense with similar RI regions, but garnet is isotropic and corundum is not |
| Blue-violet stone: tanzanite vs iolite | Refractometer reading | Specific gravity | Both strongly pleochroic, but their RI and SG ranges sit far apart |
| Blue stone: sapphire vs tanzanite | RI region | Pleochroic colours | The RI gap is wide; pleochroism confirms character when the stone is mounted |
Reading the refractometer: RI, birefringence and optic character as three different answers
One refractometer session yields three distinct results: the refractive index value, the double refraction between shadow edges, and the optic character inferred from how those edges move.
Keep the three concepts separate in your notes. The critical angle gives you the RI value itself. Doubly refracting stones can show two shadow edges, and the gap between them is the birefringence, a property with its own reference ranges. How the edges behave as you rotate the stone reveals optic character: isotropic stones show one stationary edge, while anisotropic stones show edges that move, and the pattern distinguishes uniaxial from biaxial behaviour.
Know the method's limits as firmly as its values. Stones polished only on a curved surface are read by the spot method, which gives an approximate value sufficient to place a stone in a family but not to pin a precise figure. Stones with RI above the instrument's working range show only a shadow edge near the top of the scale, which is itself diagnostic information, and it is a reason your separation tree must not depend on a single reading.
Specific gravity as the cross-check that survives mounted and out-of-range stones
Specific gravity, measured by hydrostatic weighing, is a second independent property. It resolves cases where refractometer readings are approximate, unavailable, or shared by more than one candidate stone.
The hydrostatic method weighs a stone in air and again fully immersed, and the SG equals the air weight divided by the difference between the two weighings. Trace a clean example: a loose stone weighs 4.00 carats in air and 3.00 carats immersed, so the difference is 1.00 and the SG is 4.00 divided by 1.00, that is 4.00, which sits where corundum and some garnets do. The arithmetic is simple; the discipline is recording both weighings so the calculation can be checked.
Respect the limits of the method when you interpret results. Small stones produce a small difference between weighings, so the derived SG carries wide uncertainty, and an assembled or mounted piece cannot be weighed meaningfully at all. Treat SG as the property that confirms a separation already suggested by optics or RI, and as a strong independent check on loose stones of adequate size, rather than as a universal first move.
Worked scenario one: a red stone that colour and one reading mislead
A red stone with a spot reading near the top of the refractometer scale can be corundum or a garnet. The resolving readings are optic character, supported by SG and ultraviolet behaviour.
Scenario: a red, oval-cut stone. The plausible mistake is to stop at colour plus a single spot reading around 1.76, call it ruby, and move on. That reading is approximate, and almandine garnet occupies a similar or higher RI region with an SG near 4.00, close to corundum's. On paper, both candidates now survive the first test, and colour describes both equally well.
The better decision is to test anisotropy next. Corundum is uniaxial and doubly refracting, showing distinct shadow edges and movement on the refractometer, and characteristic behaviour between crossed polars; garnet is singly refractive. Ruby also shows a strong red reaction under long-wave ultraviolet, a feature worth checking where a scenario permits. Recording which reading eliminated which candidate, and why garnet survived the first one, is exactly the reasoning the separation-tree method is meant to train.
Worked scenario two: a blue-violet stone where pleochroism alone is not enough
Tanzanite, iolite and sapphire can all present pleochroism in blue-violet material. The reliable separation runs through the refractometer's RI region first, then specific gravity, with pleochroic colours as confirmation.
Scenario: a blue-violet stone described as strongly pleochroic. The tempting shortcut is to name tanzanite on the strength of that description. The mistake is that iolite, long nicknamed the water sapphire, is also strongly pleochroic, so a written description of strong pleochroism does not separate them. A second tempting shortcut, assuming any blue stone is sapphire, fails in the opposite direction.
The better sequence places the stone by its numbers: tanzanite sits near 1.69 to 1.70 with an SG around 3.35, iolite near 1.54 to 1.55 with an SG around 2.58, and sapphire near 1.76 to 1.77 with an SG around 4.00. One RI reading removes two of the three candidates immediately; SG confirms the survivor. Pleochroic colours, noted last, become a confirmation tool rather than the decision itself, which is the correct order of evidence in any scenario.
The Australian context: training observation-first habits with opal and local gemstones
The GAA's education places visible emphasis on opal and Australian gemstones. Treat these as observation subjects: assembled and treated opal questions are answered by what a careful written description shows, not by numbers.
For assembled opal, the discriminating observations sit at the edges of the stone. A doublet or triplet shows a join line at the girdle or side profile, a difference in surface lustre between the cap and the body, and often a colour pattern that reads differently through the cap. Solid opal presents one continuous material from table to girdle. Practise by reading written stone descriptions and naming exactly which sentence carries the evidence, so your eye is trained on the location of the clue, not just its existence.
Extend the same habit to the other gemstones Australia is known for, such as sapphire and the gems catalogued in GAA's editorial material on important gemstones of Australia. For each, write one paragraph describing the observations that would identify it in a scenario: optic character, colour phenomena, and typical inclusions or growth features. This ties your study to the material the association itself highlights, without depending on memorised constants alone.
A separation-card exercise, a self-check rubric and an adaptable study sequence
Build separation cards from a consistent reference table, check yourself against a fixed rubric, then run a phased sequence that ends with mixed scenarios where the stone, not the topic, drives the method.
Exercise: using a single consistent reference source, such as the GAA's published booklet of refractive index and specific gravity values for commercially known gemstones, choose twelve commercial species and make one card each. Record the RI range, double refraction, optic character, SG and key pleochroism, then write the discriminating pair that separates the stone from each of its two closest neighbours in the table. Self-check rubric: you can name a neighbour that overlaps in at least one property for every card; each card names a second property that breaks the tie; you have written the spot-method and small-stone caveats where they apply; and you can state the sequence of readings without looking. A useful learning milestone, not a passing prediction, is explaining all twelve separations aloud in one sitting.
Adaptable preparation sequence: first, refresh the property definitions and instrument principles until each concept can be defined in one sentence. Second, build the card deck above. Third, write separation trees for the ambiguous groups. Fourth, spend focused time on opal and Australian gemstones as observation subjects. Fifth, run mixed written scenarios in random order, forcing yourself to name the reading that eliminated each rejected candidate. For enrolment, fees and administrative details of the FGAA pathway, check directly with the Gemmological Association of Australia at gem.org.au, as this guide covers study method only.
- Readiness check one: for any stone on your cards, you can state which reading would eliminate its closest neighbour first, and why.
- Readiness check two: you can calculate a hydrostatic SG from two weighings and state when the result is too uncertain to rely on.
- Readiness check three: given a written opal description, you can point to the sentence that distinguishes solid opal from an assembled stone.
- Readiness check four: in a mixed scenario, you name the evidence order, numbers before colour, confirmation last, without prompting.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.