How to tell if garnet is real
Real garnet is cubic: one refractometer reading, 1.714 to 1.888 on GIA's range, hardness 6.5 to 7.5. Glass reads 1.700 or less. A garnet-glass doublet passes.
To tell if garnet is real, use a refractometer: garnet is cubic and gives one reading, 1.714 to 1.888 on GIA's range. Glass also gives one reading, but at 1.700 or below on the International Gem Society's figures, so one reading under 1.71 is not garnet. Ruby gives two. A garnet-topped glass doublet passes, and needs a lens.
In short
- Real garnet is one of several species. GIA calls garnet a set of closely related minerals forming a group; the IMA garnet report of 2013 placed 14 species in the garnet group, among them pyrope, almandine, spessartine, grossular, andradite and uvarovite. The IMA list has rows for those species and none for garnet, rhodolite, hessonite, tsavorite or demantoid, by script.
- The 1.71 line. Garnet gives one refractometer reading, 1.714 to 1.888 on GIA’s range; the six Handbook garnet sheets used here print 1.714 to 1.887. Glass gives one reading from 1.470 to 1.700 on the International Gem Society’s figures. The gap is 0.014. One reading below 1.71 means not garnet.
- Where the line fails: spinel. The Handbook spinel sheet prints one index, n = 1.719, between pyrope’s 1.714 and grossular’s 1.734, and its colours include red. One reading above 1.71 is consistent with garnet, not proof of it. On a crystal, the form helps: spinel octahedra against garnet dodecahedra and trapezohedra.
- A documented failure from GIA’s journal. In Gems & Gemology, Fall 2023, a GIA gemmologist described a 2.54 ct stone that first presented as natural red garnet: a pyrope-almandine crown fused to glass, the garnet’s natural inclusions masking the glass beneath. Some apparent crystals were gas bubbles, easily missed without high magnification. The crown passes the refractometer; the lens has to catch it.
- Ruby is a different species, and the scale is weak. Ruby gives two readings; the rule is on corundum. A 20.00 g garnet reads 14.24 to 15.18 g in water on GIA’s range, and glass 11.30 to 15.56 g, so weighing cannot clear glass. Method: how to measure specific gravity at home.
| What it could be | Source | Readings on a refractometer | Against the 1.71 line | Crystal form on the sheet | Hardness | 20.00 g piece in water |
|---|---|---|---|---|---|---|
| Garnet, any species | GIA | One: 1.714 to 1.888 | At or above | Dodecahedra, trapezohedra (all six garnet sheets) | 6.5 to 7.5 | 14.24 to 15.18 g |
| Pyrope | Handbook sheet | One: 1.714 | Above, by 0.004 | Dodecahedron or trapezohedron, to 20 cm | 7 to 7.5 | 14.42 g |
| Almandine | Handbook sheet | One: 1.830 | Above | Dodecahedra or trapezohedra, to 1 m | 7 to 7.5 | 15.37 g |
| Spessartine | Handbook sheet | One: 1.800 | Above | Dodecahedra or trapezohedra, to 10 cm | 7 to 7.5 | 15.23 g |
| Grossular; andradite; uvarovite | Handbook sheets | One: 1.734; 1.887; 1.865 | Above | Dodecahedra, trapezohedra | 6.5 to 7 | 14.44 g; 14.74 to 14.87 g; 14.69 to 14.75 g |
| Spinel | Handbook sheet; GIA | One: 1.719 (sheet); 1.718 (GIA) | Above: the line fails | Octahedra, to 30 cm | 7.5 to 8 | 14.44 to 15.12 g |
| Glass | IGS glass article | One: 1.470 to 1.700 | Below: not garnet | None; mould marks, swirls, bubbles | 5 to 6 | 11.30 to 15.56 g |
| Garnet-topped glass doublet | GIA, G&G Fall 2023 | Reads as its garnet crown on the table | Above: the line fails | Not a crystal | Crown only | Not worked: two materials |
| Ruby (corundum) | GIA ruby page | Two: 1.762 to 1.770 | Two readings: see the corundum page | Not a garnet form | 9 | 15.00 g (SG 4.00) |
How to tell if garnet is real: five checks, in order
Start with a 10x lens; finish, if you must, at a gem laboratory. Each step costs more than the one before. Day one needs a lens, a lamp, a scale reading to 0.01 g with a thread to hang the piece in water, and, for a cut stone, the use of a refractometer whose scale reaches the reading you expect: GIA’s garnet range runs to 1.888, and almandine’s sheet value is 1.830.
- Form, on a crystal or matrix piece. All six Handbook garnet sheets used here describe dodecahedra or trapezohedra; the spinel sheet describes octahedra. The spinel sheet adds that spinel is rarely dodecahedral, so form is evidence, not proof.
- Lens. The International Gem Society says glass usually shows swirl marks and round bubbles, and moulded glass may show mould marks and rounded facet edges. On a cut stone, look below the table as well: in GIA’s doublet case the giveaway bubbles sat just beneath the garnet’s own inclusions.
- Refractometer. Apply the 1.71 line, below. Garnet is cubic and gives one reading; two readings mean another species, such as ruby.
- Scale, for loose stones and rough only. It narrows garnet against garnet; it cannot clear glass. Hardness is a test for rough only, never a cut stone; see how to test mineral hardness properly.
- Laboratory, for assembled stones you suspect but cannot see, and for laboratory-grown garnets.
The finish state is one of four labels: garnet, species unnamed; garnet, species named by a sheet value and form; not garnet, named (glass, spinel, ruby); or untested. The identification pages are indexed at the collecting guides.
Is my garnet real or glass? The 1.71 line
The 1.71 line: one refractometer reading below 1.71 means the stone is not garnet. GIA gives garnet 1.714 to 1.888 and no birefringence. The International Gem Society glass article gives glass a single index from 1.470 to 1.700. The 0.014 between them is the line. The Handbook sheets put the six garnets at pyrope 1.714, grossular 1.734, spessartine 1.800, almandine 1.830, uvarovite 1.865 and andradite 1.887, all on the garnet side.
Why the scale cannot do this job. GIA’s garnet specific gravity, 3.47 to 4.15, gives 14.24 to 15.18 g in water for a 20.00 g piece. Glass, at 2.30 to 4.50 on the gem society’s figures, gives 11.30 to 15.56 g, covering the whole garnet band. A heavy glass can weigh exactly like almandine. The refractometer and the lens are the tools against glass.
The line is one-sided. It rules glass out below 1.71; it does not prove garnet above it. Spinel sits inside the garnet range, and a doublet with a garnet crown reads as garnet. A stone that gives no reading at all may be above the top of the instrument’s scale; record that as no reading, not as a result. General imitations are on how to tell if a crystal is fake.
Garnet, ruby or spinel? Where one reading is not enough
Ruby is a separate species, and the corundum page owns that test. GIA’s ruby page gives two readings, 1.762 to 1.770, with birefringence 0.008 to 0.010. The full ruby-or-garnet rule, with the Handbook values for almandine, spessartine and grossular, is on the corundum page, and is not repeated here. Hardness separates them too, 9 against 6.5 to 7.5, but a scratch test damages a cut stone.
Spinel is the documented failure of the 1.71 line. The Handbook spinel sheet prints n = 1.719, isotropic, hardness 7.5 to 8 and density 3.6 to 4.1, rising with iron and zinc; GIA gives 1.718 and 3.60. That is one reading, inside the garnet range, from a stone whose sheet colours include red. Crystal form usually separates them on rough: spinel octahedra, garnet dodecahedra and trapezohedra, though the spinel sheet allows rare dodecahedra. On a cut red stone reading near 1.72, the honest label is garnet or spinel until a laboratory says which.
Which garnet is it? A reading names a direction, not a species, because garnets are often mixtures; GIA says many garnets are chemical mixtures of two or more species. The arithmetic is worked on almandine and spessartine; the pink-silicate question on is rhodonite a garnet.
Garnet-topped doublets and synthetic garnet: what GIA and the gem society document
The garnet-and-glass doublet passes the refractometer. In Gems & Gemology, Fall 2023, a GIA staff gemmologist described a 2.54 ct round stone that presented as natural red garnet. Under the microscope it showed transparent crystals and epitaxial needles, typical garnet inclusions; some of the apparent crystals were gas bubbles. Testing identified pyrope-almandine garnet fused with manufactured glass. As with most garnet-topped doublets, GIA wrote, the garnet formed the crown and glass most of the pavilion, so the garnet’s inclusions masked the glass.
What that means for the steps above. A refractometer reads the table facet, which is garnet, so step 3 passes it. Step 2 is the check that can catch it, and GIA says the distinction can be easily missed without careful examination under high magnification. Treat a cut red stone in an old setting, where the side cannot be seen, as untested.
Synthetic garnets are not garnet species. The International Gem Society says yttrium aluminium garnet (YAG) and gadolinium gallium garnet (GGG) are laboratory materials, that synthetic garnet was the main diamond simulant before cubic zirconia in the late 1970s, and that dopants widen their index and density ranges. It names the Proteus treatment, a thin metal surface layer on a few almandine-pyrope stones, as the only regular garnet treatment. This page gives no buying advice. Nothing on this site is offered for sale; the wanted list is the only commercial route.