Key properties
| Chemical formula | MgAl2O4 |
| Crystal system | Cubic |
| Mohs hardness | 8 |
| Toughness | Good |
| Refractive index | 1.712-1.736 |
| Optic character | Singly refractive |
| Dispersion | 0.0200 |
| Specific gravity | 3.58-3.98 |
| Cleavage | Poor (octahedral), not seen in gems |
| Fracture | Conchoidal to uneven |
| Habit | Octahedral crystals, often sharp and gemmy, frequently twinned (spinel-law contact twins); waterworn pebbles in alluvial deposits |
| Lustre | Vitreous; polish: Vitreous to subadamantine |
| UV fluorescence | Red, pink and orange stones glow weak to strong red or orange-red in LW (chromium), weaker in SW; blue, violet, green and black stones are inert |
| Pleochroism | None |
| Cause of colour | Chromium for red to pink, iron for blue and green, cobalt for the rare vivid cobalt-blue, iron-chromium mixtures for orange and purple |
| Typical inclusions | Minute octahedral crystals singly or in fingerprint strings, negative crystals, iron-stained fractures, zircon crystals with tension halos; Burmese stones may show dense reflective 'silk' |
| Stability | Stable to light and chemicals; intense heat may fade lighter colours |
| Heat sensitivity | Not heat sensitive in normal wear |
| Care and cleaning | Ultrasonic and steam usually safe; warm soapy water always safe |
| Birthstones | August (added 2016, alongside peridot); 22nd wedding anniversary |
| Availability | Reds and blues limited to scarce; black plentiful; the gem is rising fast in recognition after centuries in ruby's shadow |
| Recognition | Historically confused with ruby and sapphire; now increasingly known as a gem in its own right and an August birthstone |
Varieties (3)
Red Spinel
| Colour | red through slightly purplish red to orange-red |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Rare 4- or 6-ray asterism (star spinel); rare colour change (greyish blue to purple) |
| Appearance | Pure hot red to raspberry without ruby's secondary tones; the best Mogok stones have a glowing, almost internal flame |
| Trade names | flame spinel (orange-red) • rubicelle (orange-red, outmoded) • Jedi spinel (vivid pink-red without dark tones, Myanmar-Vietnam) |
| Misnomers | balas or balas ruby • spinel ruby • almandine spinel (historic names for red spinel sold as ruby) |
| Typical sizes | 1 to 5 ct common; 10 ct plus fine stones rare |
| Cutting styles | Faceted ovals, cushions, emerald cuts; calibrated rounds; rare stars as cabochons |
| Key separations | vs ruby: spinel SR at RI 1.718 vs ruby DR 1.762-1.770; vs pyrope garnet: garnet RI about 1.73-1.76, spectrum and inclusions; vs rubellite: tourmaline DR and RI 1.62-1.64; vs red glass: bubbles; octahedral inclusions fingerprint natural spinel |
| Sources (localities) | Myanmar (Mogok) • Tajikistan (Kuh-i-Lal, historic Badakhshan mines) • Sri Lanka • Vietnam (Luc Yen) • Tanzania (Mahenge, Tunduru) • Madagascar • Pakistan (Hunza) |
| Availability | Limited; fine red spinel is genuinely scarce and prices have risen sharply |
| Market notes | The 2007 Mahenge neon pink-red find and 'Jedi' spinels made spinel a collector star; historic red spinels (Black Prince's Ruby, Timur Ruby in the British crown jewels) supply the romance |
| Care notes | Ultrasonic and steam usually safe; warm soapy water fine |
| In short | Red spinel is the chromium-red lookalike that filled royal coffers as ruby for centuries — today prized on its own merits, with Mogok and Mahenge stones leading the market. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–3.00 | $150.00 – $1500.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $600.00 – $5000.00 | fine to top (Mogok, Mahenge, Jedi) | 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.
Blue Spinel
| Colour | greyish to violetish blue, rarely vivid cobalt blue |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Rare colour change (greyish blue daylight to purple incandescent); rare asterism |
| Appearance | Steely grey-blue to sapphire-like violetish blue; most natural blue spinel is muted, with cobalt-blue stones electric by comparison |
| Trade names | cobalt blue spinel (rare vivid stones) • gahnospinel for zinc-rich blues and greens |
| Misnomers | sapphire spinel and sapphirine (historic misnomers for blue spinel sold as sapphire) |
| Typical sizes | 1 to 5 ct typical; larger fine stones scarce |
| Cutting styles | Faceted ovals, cushions, emerald cuts; calibrated sizes common |
| Key separations | vs sapphire: spinel SR 1.718 vs sapphire DR 1.762-1.770; vs tanzanite: tanzanite trichroic, RI 1.69-1.70; vs iolite: RI 1.54-1.55 with strong trichroism; vs synthetic blue spinel (flame-fusion): RI about 1.727, anomalous double refraction (snakeskin), cobalt colour-filter reaction red; strong 460 nm iron band marks natural blue |
| Sources (localities) | Sri Lanka • Myanmar (Mogok) • Vietnam (Luc Yen) • Tanzania • Madagascar • Tajikistan • Pakistan |
| Availability | Limited; vivid natural cobalt blue extremely rare |
| Market notes | Muted grey-blues are affordable; cobalt blue spinel commands ruby-class prices per carat |
| Care notes | Ultrasonic and steam usually safe |
| In short | Blue spinel runs from affordable steely grey-blue to the electrifying, extremely rare cobalt blue — a singly refractive sapphire alternative with its own collector market. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–3.00 | $60.00 – $600.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $300.00 – $3000.00 | fine to top | 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.
Black Spinel
| Colour | black, often with submetallic sheen |
| Transparency | variable |
| Clarity type | n/a; usually opaque |
| Phenomena | Rare 4- or 6-ray asterism in dark star spinel |
| Appearance | Mirror-black with a bright vitreous to submetallic polish — crisper than onyx, harder than obsidian, heavier than glass |
| Trade names | black spinel; ceylonite or pleonaste for the iron-zinc black species material |
| Misnomers | often sold generically as 'black gemstone' alongside onyx, obsidian and hematite — always test |
| Typical sizes | Calibrated melee to 10 ct plus; abundant beads and cabochons |
| Cutting styles | Faceted rounds, ovals and beads; cabochons; popular in menswear as brilliant-cut black accents |
| Key separations | vs black onyx (dyed chalcedony): spinel much higher RI and SG, no parallel banding; vs obsidian: glass conchoidal texture, SG about 2.35; vs black diamond: diamond RI 2.417 and adamantine; vs black tourmaline: RI 1.63-1.68; vs hematite: red streak and metallic lustre |
| Sources (localities) | Thailand (Chanthaburi / Bo Rai) • Cambodia • Australia • Vietnam • Sri Lanka • Myanmar |
| Availability | Plentiful and inexpensive |
| Market notes | A favourite substitute for black diamond in fashion jewellery at a fraction of the price |
| Care notes | Ultrasonic and steam safe; durable in wear |
| In short | Black spinel is the opaque iron-rich member, prized in fashion jewellery for a mirror-bright, facet-crisp black at modest cost. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–5.00 | $5.00 – $80.00 | commercial | 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 #7 · gem #41 · gem #199
Related materials: Corundum · Diamond · Opal · Quartz · Topaz · Tourmaline
Identification wizard · All 83 materials · Singapore gem labs