Key properties
| Chemical formula | Al2O3 |
| Crystal system | Trigonal |
| Mohs hardness | 9 |
| Toughness | Excellent |
| Refractive index | 1.762-1.770 |
| Birefringence | 0.0080-0.0100 |
| Optic character | Uniaxial negative (-) |
| Dispersion | 0.0180 |
| Specific gravity | 3.95-4.10 |
| Cleavage | None; parting may appear on heavily twinned crystals |
| Fracture | Conchoidal to uneven |
| Habit | Barrel-shaped, tabular or bipyramidal hexagonal prisms with rhombohedral and pinacoidal faces; frequently recovered as waterworn alluvial pebbles |
| Lustre | Vitreous to subadamantine; polish: Vitreous to subadamantine |
| UV fluorescence | Ruby: strong crimson red in LW (chromium, quenched by iron in Thai and basalt-born stones); most blue, green and yellow sapphire inert; colourless and some pink stones inert to moderate red-orange in LW |
| Pleochroism | Dichroic, moderate to strong; ruby shows purplish red and orangy red, blue sapphire violetish blue and greenish blue, yellow varieties two tones of yellow; colourless stones none |
| Cause of colour | Trace chromophores in colourless Al2O3: chromium for red to pink, iron-titanium intervalence charge transfer for blue, iron alone for yellow and green, chromium plus iron for orange and padparadscha, colour centres (some fading) in yellow-orange |
| Typical inclusions | Rutile silk in three sets intersecting at 60 degrees, fingerprint and healed fractures, zircon crystals with dark tension halos, boehmite, hexagonal growth lines, straight angular colour zoning, polysynthetic twinning striations |
| Stability | Very stable to light, heat and everyday chemicals; boron-bearing soldering flux and pickle will etch the polished surface |
| Heat sensitivity | Not heat sensitive in normal wear or cleaning; direct torch heat can damage fracture-filled stones |
| Care and cleaning | Ultrasonic and steam cleaning usually safe for untreated and heat-treated stones; never for glass-filled or oiled stones. Warm soapy water is always safe |
| Birthstones | Ruby: July (15th and 40th anniversaries); sapphire: September (5th and 45th anniversaries) |
| Availability | Plentiful in commercial grades and melee; fine unheated stones over 5 ct, and Kashmir or Burmese ruby and sapphire, are scarce to rare |
| Recognition | Excellent; ruby and sapphire are among the most recognised gems in every market |
Varieties (6)
Ruby
| Colour | red to orangy or purplish red |
| Transparency | transparent |
| Clarity type | Type II; eye-clean stones exist but most fine ruby carries minor inclusions |
| Phenomena | Asterism (6-ray star ruby from rutile silk); 12-ray stars rare; chatoyancy very rare |
| Appearance | Saturated medium-dark red that glows under daylight and incandescent light alike; the finest stones show a inner fire from chromium fluorescence rather than a flat surface colour |
| Trade names | Burmese ruby • pigeon's blood (vivid red, historically Mogok) • Siama or Siam (dark Thai-Cambodian red) • Ceylon (lighter Sri Lankan red) |
| Misnomers | balas ruby (red spinel) • Cape ruby (pyrope garnet) • Arizona ruby (pyrope) • Australian ruby (garnet) • ruby spinel (spinel) • rubellite (red tourmaline) |
| Typical sizes | Melee to about 5 ct commonly; clean stones over 5 ct rare and expensive; museum stones over 50 ct exceptional |
| Cutting styles | Faceted in ovals, cushions, emerald cuts and mixed cuts; translucent silky material as cabochons for star rubies; native-cut Asian stones often retain weight |
| Key separations | vs red spinel: RI 1.762-1.770 DR vs 1.718 SR, and spinel octahedral inclusions; vs rubellite tourmaline: RI 1.62 and strong dichroism vs 1.76; vs red garnet: garnet SR, no dichroism, higher SG in almandine; vs red glass: bubbles and single RI; vs flame-fusion synthetic: curved striae, plumlike flux inclusions in flux synthetics |
| Sources (localities) | Myanmar (Mogok) • Mozambique (Montepuez) • Thailand (Bo Rai, Chanthaburi) • Cambodia (Pailin) • Sri Lanka • Tanzania (Songea, Winza) • Madagascar • Vietnam (Luc Yen) • Afghanistan • Tajikistan |
| Availability | Commercial grades plentiful from Mozambique; fine Burmese-type over 1 ct scarce; unheated material rare and commands premiums |
| Market notes | Origin (Myanmar vs Mozambique) and absence of treatment drive price more than any other coloured stone; ruby holds the per-carat auction records for coloured gems |
| Care notes | Ultrasonic safe unless fracture-filled; never steam or ultrasonic clean glass-filled rubies; disclose filler before any bench work |
| In short | Ruby is red corundum, coloured by chromium, and for centuries the most prized coloured gemstone. Fine stones combine saturated red with red fluorescence that makes the colour glow. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–1.00 | $150.00 – $2500.00 | commercial to fine | 2026-10-26 |
| 1.00–3.00 | $800.00 – $12000.00 | fine | 2026-10-26 |
| 3.00–10.00 | $5000.00 – $100000.00 | fine to top collectible (Burmese unheated) | 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 Sapphire
| Colour | violetish blue to greenish blue |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Asterism (star sapphire); rare colour change blue to purple; very rare chatoyancy |
| Appearance | Velvety cornflower to deep royal blue; the best stones keep saturation without turning blackish in incandescent light |
| Trade names | Kashmir (velvety cornflower, finest) • royal or Burmese blue • Ceylon (lighter brilliant blue) • Montana (steely blue) • Australian (dark inky blue) • geuda (milky Sri Lankan rough that heats to blue) |
| Misnomers | oriental aquamarine (light greenish blue sapphire) • Brazilian sapphire (blue tourmaline) • water sapphire (iolite) |
| Typical sizes | Melee to over 10 ct common; clean 10 ct plus fine stones rare and valuable |
| Cutting styles | Faceted ovals, cushions, emerald cuts, rounds; silky material cut as cabochon star sapphires |
| Key separations | vs blue spinel: RI 1.762-1.770 DR vs 1.718 SR; vs iolite: RI 1.54-1.55 and very strong trichroism; vs tanzanite: RI 1.69, biaxial, strong trichroism; vs benitoite: higher RI and dispersion; vs kyanite: RI and variable hardness; vs synthetic: curved striae (flame-fusion) or flux veil; colour-zone immersion separates diffused stones |
| Sources (localities) | Sri Lanka (Ratnapura) • Madagascar (Ilakaka, Ambatovy) • Kashmir (Padar) • Myanmar (Mogok) • Australia • Thailand-Cambodia • Montana (Yogo Gulch) • China (Shandong) • Nigeria • Laos |
| Availability | Plentiful in heated commercial grades; unheated fine and Kashmir stones scarce to rare |
| Market notes | Kashmir origin carries the highest premium in coloured stones; heat treatment is near universal and must be assumed unless certified unheated; beryllium diffusion must be disclosed |
| Care notes | Ultrasonic and steam usually safe; avoid for heavily fractured or filled stones |
| In short | Blue sapphire is corundum coloured by iron and titanium and the most traded coloured gemstone after, and often alongside, diamond. Kashmir cornflower, Burmese royal and Ceylon blue define its classic origins. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–1.00 | $50.00 – $600.00 | commercial to fine | 2026-10-26 |
| 1.00–3.00 | $150.00 – $3000.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $1000.00 – $50000.00 | fine to top (Kashmir-type) | 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.
Colourless Sapphire
| Colour | colourless (white sapphire) |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Rare asterism |
| Appearance | Bright, completely colourless and sparkling, but without the dispersion or adamantine lustre of diamond; a natural, honest diamond substitute |
| Trade names | white sapphire • leuco sapphire |
| Misnomers | diamond substitute claims should be qualified; it is sapphire, not any form of diamond |
| Typical sizes | Melee to several carats readily available |
| Cutting styles | Round brilliants and fancy shapes to imitate diamond cuts; also calibre and melee |
| Key separations | vs diamond: RI 1.76-1.77 with DR vs 2.417 SR, thermal probe, dispersion; vs CZ: SG 4.0 vs 5.8 and thermal tester; vs glass: inclusions and single RI; vs colourless spinel: DR vs SR |
| Sources (localities) | Sri Lanka • Madagascar • Tanzania • Australia • Myanmar |
| Availability | Good; calibrated melee and matched pairs available |
| Market notes | Sold as a natural affordable diamond alternative; also heavily produced by flame-fusion synthesis, so verification matters |
| Care notes | Ultrasonic and steam safe |
| In short | Colourless or white sapphire is pure corundum without chromophores, used widely as a natural diamond alternative in melee and side stones. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.30–1.00 | $20.00 – $150.00 | commercial to fine | 2026-10-26 |
| 1.00–3.00 | $60.00 – $250.00 | 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.
Black Star Sapphire
| Colour | near-black, often dark greenish blue in strong transmitted light |
| Transparency | variable |
| Clarity type | Type II; usually translucent rather than transparent |
| Phenomena | Asterism, typically a sharp 6-ray star; 12-ray stars rare |
| Appearance | Near-black groundmass carrying a bright silver or gold rayed star that glides across the dome |
| Trade names | black star sapphire; gold-star and silver-star varieties by ray colour |
| Misnomers | some cheap black stars are diffusion-treated or even synthetic; natural stars should be confirmed |
| Typical sizes | Common from 2 to 20 ct; large cabochons readily available |
| Cutting styles | High-dome cabochons oriented on the c-axis so the star centres on the dome |
| Key separations | vs synthetic star sapphire: perfect star sitting on the surface, curved striae, glass bubbles; vs diffusion-treated star: star confined to a thin surface layer visible as colour rim in immersion; vs black spinel or schorl: RI and magnification |
| Sources (localities) | Thailand (Chanthaburi) • Australia • Sri Lanka • Cambodia • West Africa |
| Availability | Plentiful; the largest and cheapest star sapphires on the market are usually Thai diffusion-treated |
| Market notes | Huge natural black stars over 100 ct exist; price per carat is modest unless the star is sharp and natural |
| Care notes | Warm soapy water; avoid re-polishing diffusion-treated stones, which strips the star |
| In short | Black star sapphire is iron-rich, silk-bearing corundum cut in dome cabochons to show a moving white or gold star. Most large commercial black stars are surface-diffusion treated. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 2.00–10.00 | $10.00 – $120.00 | commercial (mostly diffusion-treated) | 2026-10-26 |
| 10.00–50.00 | $30.00 – $250.00 | natural fine stars | 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.
Green Sapphire
| Colour | yellowish green to bluish green, often dark |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Rare asterism; rare colour change |
| Appearance | Usually deep forest or olive green, less lively than emerald or tsavorite; attractive in medium tones |
| Trade names | oriental emerald (outmoded) is sometimes misapplied; most greens are simply sold as green sapphire |
| Misnomers | oriental emerald • oriental peridot (both historic misnomers) |
| Typical sizes | 1 to 10 ct common |
| Cutting styles | Faceted ovals, cushions, emerald cuts; some cabochons |
| Key separations | vs emerald: RI 1.76 vs 1.58 and different inclusions; vs peridot: RI, birefringence and spectrum (450-470 iron trio); vs tsavorite: SR garnet RI 1.74; vs chrome tourmaline: RI 1.62-1.64 and spectrum |
| Sources (localities) | Australia • Thailand-Cambodia • Sri Lanka • Tanzania • Madagascar • Montana |
| Availability | Regular but limited demand; undervalued against blue sapphire |
| Market notes | Modest prices; sellers sometimes relabel green sapphire as more exotic greens, so testing matters |
| Care notes | Standard corundum care; ultrasonic safe |
| In short | Green sapphire is an iron-coloured fancy sapphire, typically deep olive to forest green. It is a hard-wearing, affordable green gem even if it lacks the brilliance of emerald or tsavorite. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–3.00 | $25.00 – $200.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $60.00 – $400.00 | 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.
Yellow Sapphire
| Colour | greenish yellow through orangy yellow |
| Transparency | transparent |
| Clarity type | Type II |
| Phenomena | Rare asterism; rare colour change |
| Appearance | Bright lemon to golden yellow with good sparkle; the best Sri Lankan stones resemble fine yellow diamond tones at a fraction of the price |
| Trade names | golden sapphire |
| Misnomers | oriental topaz • king topaz • imperial topaz (all historic misnomers for yellow sapphire) |
| Typical sizes | 1 to 10 ct common; larger clean stones found |
| Cutting styles | Faceted ovals, cushions, rounds; step cuts for larger clean material |
| Key separations | vs yellow chrysoberyl: RI 1.746-1.755 vs 1.762-1.770 and different spectrum (chrysoberyl 445 nm band); vs citrine: RI 1.544-1.553; vs yellow topaz: RI 1.61-1.64 and SG; vs yellow tourmaline: RI and SG; vs Be-diffused sapphire: colour rims in immersion and chemistry |
| Sources (localities) | Sri Lanka • Australia (strong iron spectrum) • Madagascar • Thailand • Tanzania • Montana |
| Availability | Good supply in calibrated sizes |
| Market notes | Fading stones from unstable colour centres exist; buy vivid yellow with lab confirmation. Be-diffused orange-yellows are widespread and must be disclosed |
| Care notes | Standard corundum care |
| In short | Yellow sapphire is a bright fancy sapphire coloured by iron or colour centres. Fine unheated Sri Lankan lemon-gold stones are excellent value; beware fading irradiated material and undisclosed beryllium diffusion. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–3.00 | $30.00 – $400.00 | commercial to fine | 2026-10-26 |
| 3.00–10.00 | $100.00 – $900.00 | fine unheated | 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 #4 · gem #40 · gem #68 · gem #106 · gem #161 · gem #197
Related materials: Diamond · Opal · Quartz · Spinel · Topaz · Tourmaline
Identification wizard · All 83 materials · Singapore gem labs