Key properties
| Chemical formula | Fe3Al2(SiO4)3 |
| Crystal system | Cubic |
| Mohs hardness | 7-7.5 |
| Toughness | Fair to good |
| Refractive index | 1.770-1.820 |
| Optic character | SR (singly refractive; isotropic) - anomalous double refraction common |
| Dispersion | 0.0240 |
| Specific gravity | 3.95-4.30 |
| Cleavage | None; indistinct parting in some directions |
| Fracture | Conchoidal to uneven |
| Habit | Rhombic dodecahedron and trapezohedron crystals, also rounded alluvial pebbles |
| Lustre | Vitreous, sometimes greasy on fractured surfaces; polish: Vitreous to subadamantine |
| UV fluorescence | Inert in long- and short-wave UV (iron quenches any fluorescence) |
| Pleochroism | None (isotropic) |
| Cause of colour | Ferrous iron (Fe2+) as an essential component of the formula; the strongest red of the common garnets |
| Typical inclusions | Coarse rutile needles (can produce asterism) • zircon crystals with strain halos • irregular rounded crystals of low relief • apatite • mica flakes • fingerprint-type fluid inclusions |
| Stability | Stable to light and everyday chemicals; attacked only very slightly by concentrated hydrofluoric acid |
| Heat sensitivity | Abrupt temperature change can fracture the stone; avoid torch heat during repair work |
| Care and cleaning | Ultrasonic usually safe • steamer never • warm soapy water safe |
| Birthstones | January (garnet) • Aquarius • 2nd wedding anniversary |
| Availability | Plentiful in small to medium sizes; very large clean stones less common but available |
| Recognition | Widely worn but rarely known by species name - most people simply call any red garnet 'almandite' |
Varieties
Almandite
| Colour | reddish orange to red, slightly purplish red to reddish purple, typically dark in tone |
| Transparency | transparent |
| Clarity type | Type II - typically included |
| Phenomena | Asterism in star garnet (usually four-rayed, sometimes six-rayed, from oriented rutile needles) |
| Appearance | Deep red to reddish-purple stone with a warm, slightly brownish or violet cast; tone is usually medium-dark to dark, so large dark stones are often cut shallow to keep them lively. |
| Trade names | star garnet (asteriated, Idaho state gem) • almandine |
| Misnomers | Cape ruby • Colorado ruby • Arizona ruby • Bohemian carbuncle • carbuncle (historic, any red cabochon) |
| Typical sizes | Melee up to about 10 ct common; faceted stones to 40 ct known |
| Cutting styles | Faceted in all standard shapes (often shallow for dark material) • cabochons (incl. star) • beads • carvings |
| Key separations | vs ruby: SR/DR on polariscope, RI 1.77-1.82 over limit vs ruby 1.762-1.777, ruby fluoresces red, almandite inert, almandite Fe spectrum 504/520/573 nm • vs pyrope: pyrope lower RI (near 1.74) and lighter SG • vs red spinel: spinel RI 1.718-1.733, SG ~3.60, rounded octahedral crystals • vs rubellite tourmaline: DR, lower SG, strong pleochroism • vs garnet-topped doublet: check join plane and RI difference between crown and pavilion |
| Sources (localities) | India • Sri Lanka • Madagascar • Brazil • USA (Idaho star garnets) • Austria • Greenland • Tanzania |
| Availability | Plentiful and inexpensive across all jewellery categories |
| Market notes | Historically fused to glass bases as garnet-and-glass doublets (late 1800s-early 1900s); antiques still surface. Fine transparent reds compete with rhodolite at the quality end. |
| Care notes | Remove before torch work; thermal shock can crack the stone |
| In short | Almandite is the deep red iron garnet most consumers picture when they hear 'garnet'. Dark in tone and inexpensive, it is a workhorse red stone; asteriated material from Idaho and India is cut as star garnet. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–3.00 | $8.00 – $60.00 | commercial to good | 2026-10-26 |
| 3.00–10.00 | $15.00 – $120.00 | commercial to fine | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Sources
GIA gem encyclopedia · mindat.org · Wikipedia
Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.
See also
Gemstone reference entries: gem #3
Related materials: Andradite Garnet · Grossular Garnet · Hydrogrossular Garnet · Pyrope Garnet · Rhodolite Garnet · Spessartine Garnet · Tsavorite Garnet
Identification wizard · All 83 materials · Singapore gem labs