Is rhodonite a garnet
No. Rhodonite is a triclinic manganese silicate with two cleavages at 92.5 degrees. Garnets such as spessartine are cubic, singly refractive and lack cleavage.
Is rhodonite a garnet? No. Rhodonite is a triclinic manganese silicate, hardness 5.5 to 6.5, with two perfect cleavages meeting at 92.5 degrees. Garnets are cubic, singly refractive and have no cleavage; the manganese garnet is spessartine, hardness 7 to 7.5, density 4.190. A broken edge or a polariscope separates them; a scale may not.
In short
- Different structures, different formulas. The Handbook of Mineralogy prints rhodonite as (Mn2+,Fe2+,Mg,Ca)SiO3, triclinic, and the September 2026 IMA list as CaMn3Mn(Si5O15), redefined in 2019. Spessartine is Mn2+3Al2(SiO4)3, cubic, in the garnet group. GIA describes all garnets as sharing essentially the same crystal structure, varying in composition.
- The two-plane rule: a pink stone that breaks along two flat planes meeting near a right angle is not a garnet. Rhodonite has perfect cleavage on {110} and {1-10} at 92.5 degrees and good on {001}. The spessartine and andradite sheets record no cleavage, almandine and grossular only a parting on {110}, and the International Gem Society gives garnet cleavage as none.
- The polariscope agrees. Rhodonite is biaxial, doubly refractive. The garnet sheets read isotropic, and the gem society gives garnet birefringence as none, so a transparent garnet behaves as singly refractive between crossed polars, apart from the weak anomalous effects its sheets mention.
- The scale works for spessartine and almandine, not grossular. A 20.00 g piece reads 14.40 to 14.68 g in water as rhodonite, 15.23 g as spessartine and 15.37 g as almandine. Grossular, whose sheet lists pink among its colours, reads 14.44 g, inside rhodonite's band.
- Twelve rows of the IMA list have the strict garnet formula shape X3Y2(SiO4)3. Three put manganese at X: calderite, momoiite and spessartine. Rhodonite's row has nothing of that shape. The shared prefix rhodo-, as in rhodolite garnet, is a name, not a family.
| Criterion | Rhodonite | Spessartine | Almandine | Grossular | Andradite | Does it choose? |
|---|---|---|---|---|---|---|
| Crystal system | Triclinic | Cubic | Cubic | Cubic | Cubic | Yes, if crystals are present |
| Crystals | Rough, rounded edges, tabular, to 20 cm; commonly massive | Dodecahedra or trapezohedra, to 10 cm | Dodecahedra or trapezohedra, to 1 m | Dodecahedra or trapezohedra, to 15 cm | Dodecahedra, trapezohedra, to 5 cm | Yes: the garnet sheets record dodecahedra and trapezohedra |
| Cleavage | Perfect on {110} and {1-10} at 92.5 degrees; good on {001} | None recorded | Parting on {110} | Parting on {110}, rarely observed | None recorded | Yes: the two-plane rule |
| Optical class | Biaxial (+) | Isotropic; may show weak anisotropism | Isotropic; anomalously biaxial | Isotropic; weak strain birefringence | Isotropic; typically weakly anisotropic | Yes, on a polariscope |
| Refractive index | 1.711–1.748 | 1.800 | 1.830 | 1.734 | 1.887 | Not against grossular |
| Hardness | 5.5–6.5 | 7–7.5 | 7–7.5 | 6.5–7 | 6.5–7 | Partly: ranges touch at 6.5 |
| Density, measured | 3.57–3.76 | 4.190 | 4.318 | 3.594 | 3.8–3.9 | Not against grossular |
| 20.00 g piece in water | 14.40–14.68 g | 15.23 g | 15.37 g | 14.44 g | 14.74–14.87 g | Spessartine and almandine: yes. Andradite: 0.06 g. Grossular: no |
| Pink or rose among sheet colours | Rose-pink to brownish red | Red, reddish orange; pale pink only in thin section | Deep red, red-violet | Pink, among many | No | No |
Is rhodonite a garnet? What the formulas say
No. Rhodonite and garnet are different kinds of silicate, and both reference sources show it. The Handbook of Mineralogy sheet for rhodonite prints (Mn2+,Fe2+,Mg,Ca)SiO3, triclinic. The IMA-CNMNC list of minerals gives rhodonite as CaMn3Mn(Si5O15), status Rd, redefined, 2019 s.p.; the two formulas differ, and both are shown here as each source prints them. The Handbook of Mineralogy sheet for spessartine gives Mn2+3Al2(SiO4)3, cubic, with the mineral group line Garnet group.
Method, and a count. Every line of the September 2026 IMA list, saved as text with pdftotext -layout, was scanned by script for a formula of the strict shape X3Y2(SiO4)3, one element at each site. Twelve rows match: almandine, andradite, calderite, eringaite, goldmanite, grossular, knorringite, momoiite, pyrope, rubinite, spessartine and uvarovite. Three put manganese at X: calderite, momoiite and spessartine. This is a formula-shape count, not the IMA's garnet membership list; three more rows, majorite, menzerite-(Y) and morimotoite, have a mixed site and are left out by the strict rule. GIA writes that there are more than twenty garnet species.
Rhodonite's row has nothing of that shape. Where the garnet formulas are written with three SiO4 groups, rhodonite's IMA formula is written with Si5O15. Rhodonite is also distinct from its carbonate namesake, as set out on is rhodochrosite the same as rhodonite.
The two-plane rule: why a broken edge decides
The two-plane rule: if a pink stone breaks along two flat, bright planes meeting close to a right angle, it is not a garnet. Rhodonite's sheet records perfect cleavage on {110} and {1-10}, with the angle between them 92.5 degrees, and a good cleavage on {001}, read from the rendered sheet. The gem society notes that this perfect cleavage makes transparent rhodonite delicate.
The garnet sheets record no cleavage to compete. The spessartine sheet gives only fracture, uneven to conchoidal; andradite the same; the almandine sheet a parting on {110}; and the Handbook of Mineralogy sheet for grossular a parting on {110} rarely observed. A garnet's broken surface is curved or irregular. The general skill of reading a broken surface is on cleavage or fracture.
The polariscope gives the same verdict without a break. Rhodonite is biaxial positive, doubly refractive, with birefringence 0.011 to 0.014 on the gem society's figures. The garnet sheets read isotropic, qualified by weak anisotropism or strain birefringence, and the gem society gives garnet birefringence as none. That is the test for a polished bead you do not want to chip.
Which garnet could pass for rhodonite? Grossular, on the scale
Weighing separates rhodonite from the dense garnets and fails against grossular. Worked as weight in water equals mass times one minus one over density, a 20.00 g piece reads 14.40 to 14.68 g as rhodonite (3.57 to 3.76) and 15.23 g as spessartine (4.190), a gap of 0.55 g from rhodonite's heaviest. Almandine (4.318) reads 15.37 g. Andradite (3.8 to 3.9) reads 14.74 to 14.87 g, only 0.06 g clear. Grossular, 3.594, reads 14.44 g, inside rhodonite's band.
Grossular is the one to take seriously. Its sheet lists pink among its colours, hardness 6.5 to 7 touches rhodonite's 6.5, and its index, n = 1.734, falls inside rhodonite's 1.711 to 1.748 and next to the 1.73 the gem society gives for massive rhodonite. Its density, index and hardness all fall inside or touch rhodonite's ranges; what separates them is structure: no cleavage, and isotropic. So for a pink stone the order is broken edge or polariscope first, scale second. The full home sequence is on how to tell if rhodonite is real; the method for the scale is on how to measure specific gravity at home.
Who each answer is wrong for. Rhodonite is the wrong answer for anyone holding pink or red dodecahedra or trapezohedra with no flat break: that is a garnet. Garnet is the wrong answer for anyone holding massive rose-pink material with black veins that splits along two near-square planes. Neither suits a tumbled pebble with no break and no polariscope reading; the honest label there is pink silicate, untested.
Why people ask: spessartine, rhodolite and the rose names
The overlap is in names and settings, not in mineralogy. The rhodonite sheet says its name comes from the Greek for rose. GIA calls rhodolite a purple-red garnet, and the International Gem Society describes it as a blend of pyrope and almandine with a distinctive purplish colour. Rhodolite is a garnet variety name; rhodonite is a species. They share a prefix and nothing structural.
They also share deposits. Spessartine's sheet lists rhodonite among its associates, and the rhodochrosite sheet lists rhodonite and garnet together among its metamorphic associates. The spessartine and rhodonite sheets both name Broken Hill, New South Wales, so a mixed specimen from there may carry both: test each pink area separately. The locality is on Broken Hill, and the carbonate side of manganese is on rhodochrosite.
Spessartine is the orange garnet in GIA's words, and its sheet colours run red, reddish orange, yellow and brown; pink appears there only in thin section. The identification pages are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.