Key properties
| Chemical formula | Silica-rich rhyolitic glass (about 70%+ SiO2 with Al, Na, K, Fe) |
| Crystal system | Amorphous (glass) |
| Mohs hardness | 5-5.5 |
| Toughness | Fair to good |
| Refractive index | 1.480-1.510 |
| Optic character | SR (isotropic); anomalous double refraction common from strain n/a |
| Specific gravity | 2.33-2.50 |
| Cleavage | None |
| Fracture | Conchoidal - razor-sharp edges (used for prehistoric blades) |
| Habit | Massive flows and domes; nodules (Apache tears) in perlite |
| Lustre | Vitreous (glassy); polish: Vitreous |
| UV fluorescence | Generally inert |
| Pleochroism | None |
| Cause of colour | Dark tones from iron and other transition elements plus fine particle scattering; greens, browns and mahogany reds from oxidised iron; banding from flow layering |
| Typical inclusions | Gas bubbles, crystallites, stubby needle-like microlites; oriented microcrystallite layers (golden sheen), cristobalite spherulites (snowflakes), hedenbergite nanorods (rainbow), magnetite nanoparticles (fire obsidian) |
| Stability | Stable to light; attacked by hydrofluoric acid; devitrifies to cristobalite over deep time |
| Heat sensitivity | Cracks with rapid temperature change and melts at relatively low temperature - protect from torches |
| Care and cleaning | Warm soapy water safe; ultrasonic usually safe; steam risky |
| Birthstones | None |
| Availability | Plentiful and inexpensive; decorative varieties (rainbow, snowflake) widely cut |
| Recognition | Familiar to everyone as 'volcanic glass' - Apache tears and snowflake obsidian are staples of silver jewellery |
Varieties (2)
Sheen / Snowflake Obsidian
| Colour | black to dark grey with golden or silvery sheen, iridescent rainbow bands, or white snowflake patches |
| Transparency | opaque |
| Clarity type | n/a (opaque) |
| Phenomena | sheen and iridescence (rainbow obsidian) |
| Appearance | Polished black glass animated by a floating golden or silver glow (sheen), concentric rainbow bands (rainbow), or a scatter of white snowflakes across the charcoal ground. |
| Trade names | gold-sheen obsidian • silver-sheen obsidian • rainbow obsidian • snowflake obsidian (flowering obsidian) • velvet obsidian • mahogany obsidian (red-brown, iron-stained) |
| Misnomers | mountain mahogany • Montana jet • Iceland agate • glass agate • Nevada topaz • Nevada diamond • agni mani • fire pearl (historical fancy names) |
| Typical sizes | can be very large - slabs and carving blocks; cabs in every size |
| Cutting styles | Cabochons (oriented to place the sheen at the dome apex), carvings, beads, spheres, knife blades and arrowheads |
| Key separations | vs manmade black glass: look for natural flow banding, crystallites and microlites versus mould seams and seed bubbles; vs black onyx (dyed chalcedony): onyx RI 1.535-1.539 spot and hardness 7, obsidian spot 1.49 and scratches at 5-5.5; vs jet: jet SG 1.3 and burns; vs black tourmaline: schorl is DR with striated prismatic crystal form |
| Sources (localities) | Mexico (rainbow obsidian) • USA (Oregon sheen, California, Utah Apache tears) • Iceland • Armenia • Turkey • Italy (Lipari, Sardinia) • Ethiopia • Guatemala • Japan • New Zealand • Peru • Chile |
| Availability | Plentiful and inexpensive in all phenomenal varieties |
| Market notes | Steady demand for silver jewellery and carvings; rainbow and gold-sheen command small premiums |
| Care notes | Avoid harsh knocks and thermal shock; warm soapy water |
| In short | Sheen and snowflake obsidians are black volcanic glasses animated by oriented bubbles (golden glow), nanorod layers (rainbow) or white cristobalite snowflakes. Inexpensive, plentiful, and cut as cabs, carvings and beads. Spot RI about 1.49 and hardness 5-5.5 identify them. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–20.00 | $0.50 – $6.00 | commercial | 2026-10-26 |
| 20.00–200.00 | $1.00 – $10.00 | carvings and spheres | 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 Obsidian
| Colour | uniform jet black, sometimes translucent brown or smoky on thin edges |
| Transparency | opaque |
| Clarity type | n/a (opaque) |
| Phenomena | occasionally iridescent bands (rainbow obsidian); rare chatoyancy |
| Appearance | Mirror-black glass with a mirror-bright polish; occasionally smoky translucency at thin edges. Apache tears are rounded, smoky-grey pebbles of obsidian in a whitish perlite rind. |
| Trade names | black obsidian • Apache tears (rounded translucent pebbles) • midnight lace obsidian (banded) |
| Misnomers | mountain jet • Montana jet • glass agate (historic) • 'black onyx' is frequently substituted by dyed chalcedony, not obsidian |
| Typical sizes | very large pieces available; beads and cabs in all calibrated sizes |
| Cutting styles | Cabochons, beads, carvings, blades/arrowheads, small faceted beads and 'diamond-cut' rounds |
| Key separations | vs black glass: natural flow banding, crystallites and conchoidal chips with razor edges versus mould marks; vs black onyx: RI and hardness (onyx 1.53+, H 7); vs jet: SG and hot point; vs black tourmaline: optic character and crystal habit; vs hematine (imitation hematite): streak and SG |
| Sources (localities) | Worldwide at recent volcanoes: Mexico • USA • Iceland • Italy (Lipari) • Armenia • Turkey • Ethiopia • Kenya • Guatemala • Japan • New Zealand • Chile • Peru • Indonesia |
| Availability | Plentiful and the cheapest of all black gem materials |
| Market notes | Volume material for beads, men's jewellery and objets d'art; Apache tears sold as tumbled amulets |
| Care notes | Sharp when chipped - protect edges; avoid thermal shock; warm soapy water |
| In short | Black obsidian is the plain jet-black volcanic glass: RI about 1.49, SG about 2.4, conchoidal fracture, taking a mirror polish for cabs, beads and carvings. Distinguish from black onyx and black glass by spot RI and hardness. Apache tears are its rounded pebble form. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 1.00–20.00 | $0.30 – $4.00 | commercial | 2026-10-26 |
| 20.00–500.00 | $0.50 – $5.00 | carvings | 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 #148 · gem #217
Related materials: Corundum · Diamond · Opal · Quartz · Spinel · Topaz
Identification wizard · All 83 materials · Singapore gem labs