Is obsidian a mineral?
Obsidian is not a mineral: it is natural volcanic glass with no crystal structure, typically 66 to 72% SiO2 by mass. The minerals are the crystals inside it.
No. Obsidian is not a mineral species: the USGS calls it a natural glass with no crystalline structure, and the International Gem Society a rock of about 66 to 72% SiO2, hardness 5, with no cleavage. It passes two of the four clauses of the mineral definition. The snowflakes in it are crystals of a mineral, cristobalite.
In short
- Obsidian is a glass, so it is not a mineral. The USGS describes it as a natural glass with no crystalline structure; the International Gem Society (IGS) calls it the most common form of natural glass and says it is considered a rock. The September 2026 IMA list has no row for it, and no line of the list mentions it.
- Two of four. Applied clause by clause, Tropper’s definition of a mineral gives obsidian a pass on naturally occurring and on solid, and a fail on a highly ordered atomic arrangement and on definite composition. The IGS gives it a variable composition, about 66 to 72% SiO2 in typical obsidian and about 50% in basaltic glass.
- The minerals are the things inside it. The IGS names cristobalite as the snowflake-like spherulites of snowflake obsidian and magnetite nano-crystals as the cause of rainbow and fire obsidian. Both have IMA rows: cristobalite G, 1887, Mexico, and magnetite G, 1845. The glass around them has none.
- Weight does not see the snowflakes. On its Handbook sheet cristobalite is 2.32 to 2.36; typical obsidian is 2.33 to 2.42 on the IGS figures. A 20.00 g piece of either reads about 11.4 to 11.7 g in water, so a scale cannot tell glass from the mineral grown in it.
- Natural is not the same as mineral. Obsidian forms in nature, sometimes recently: the USGS dates the ash-flow tuff of the eruption that ended in Newberry’s Big Obsidian Flow at about 1300 years. Man-made blue, green or red glass sold as rare obsidian, which the IGS warns of, is neither obsidian nor a mineral.
| Material | Source | Naturally occurring? | Ordered atomic arrangement? | Definite composition? | IMA row? | Specific gravity | A mineral? |
|---|---|---|---|---|---|---|---|
| Obsidian | USGS; IGS | Yes | No: natural glass, amorphous | No: SiO2 about 66 to 72%, variable | No | 2.25 to 3.00; typically 2.33 to 2.42 (IGS) | No |
| Basaltic glass, Pele’s hair | IGS | Yes | No: glass | No: SiO2 about 50% | No | 2.70 to 3.00 (IGS) | No |
| Apache tears | IGS | Yes | No: cores of unaltered glass | No | No | As obsidian | No: obsidian |
| Snowflakes in snowflake obsidian: cristobalite | IGS; Handbook sheet | Yes | Yes: tetragonal, pseudocubic | Yes: SiO2 | Yes: G, 1887, Mexico | 2.32 to 2.36 (Handbook) | Yes |
| Colour layers in rainbow and fire obsidian: magnetite | IGS; Handbook sheet | Yes | Yes: cubic | Yes: Fe2+Fe3+2O4 | Yes: G, 1845 | 5.175 (Handbook) | Yes |
| Coloured glass sold as rare obsidian | IGS obsidian and glass articles | No: manufactured | No: glass | No | No | 2.30 to 4.50 for glass in general (IGS) | No |
Is obsidian a mineral? No, it is a natural glass
Obsidian is a natural glass, and a glass is not a mineral because it has no ordered atomic structure. The USGS page on the Big Obsidian Flow, Newberry Volcano says it plainly: obsidian is a natural glass with no crystalline structure. The International Gem Society obsidian article calls obsidian the most common form of natural glass, gives its crystallography as amorphous, and says that, formed by volcanism, it is considered a rock.
The IMA list agrees by omission. A script over the September 2026 list, matching names and then scanning every line of the text, finds no row for obsidian and no line that mentions it. The list is of mineral species, and obsidian is not one. The live crystal vs mineral page names obsidian among the label words that have no row; this page shows why.
What obsidian is, in figures. The IGS gives a variable composition, SiO2 approximately 66 to 72% plus oxides of calcium, sodium, potassium and others, with basaltic glass at about 50%; hardness 5, or 6 for basaltic glass; specific gravity 2.25 to 3.00, typically 2.33 to 2.42; one refractive index, 1.48 to 1.51. These are the gem society’s figures; the Handbook of Mineralogy has no obsidian sheet (the request returned 404), so there is no sheet value to quote.
Why obsidian is not a mineral: the definition, clause by clause
Obsidian passes two of the four clauses and fails two. Tropper, Introduction to the Mineralogical Sciences (EOLSS) defines a mineral as a naturally occurring homogeneous solid with a definite, but not fixed, chemical composition and a highly ordered atomic arrangement, and says the ordered arrangement indicates that a mineral is in a crystalline state. The same four-part test is worked on quartz, cultured quartz and silica glass on is quartz a mineral; here it is worked on obsidian.
- Naturally occurring: pass. It forms in volcanic eruptions; the IGS finds it in areas of present and former volcanic activity.
- Solid: pass. Hardness 5 on the IGS figures; the gem society records it made into scalpels, arrowheads and knives.
- Highly ordered atomic arrangement: fail. No crystalline structure (USGS); amorphous (IGS).
- Definite composition: fail. The chapter requires atoms in specific ratios, allowing variation by solid solution. Obsidian’s SiO2 runs from about 50% in basaltic glass to 72%, with other oxides making up the rest: a range, not a ratio.
The decision rule: a material that fails the ordered-arrangement clause is not a mineral, however natural it is. That one clause is enough. Obsidian also fails a second, which is why it is called a rock rather than a glassy mineral.
Does obsidian contain minerals? Snowflakes and rainbow layers
Yes: some obsidian contains crystals, and those crystals are minerals. The IGS names two. The white snowflakes of snowflake obsidian are spherulites of cristobalite, which it calls protogenic silica minerals crystallising in obsidian. The colours of rainbow and fire obsidian come from inclusions of magnetite nano-crystals, thinner in fire obsidian than in rainbow. It also lists crystallites and needles as inclusions that point to a natural origin.
Both are IMA rows. Cristobalite is SiO2, G, 1887, Mexico; the Handbook of Mineralogy cristobalite sheet gives it tetragonal, pseudocubic, hardness 6 to 7, density 2.32 to 2.36. Magnetite is Fe2+Fe3+2O4, G, 1845, recorded by its sheet as strongly magnetic. The IGS adds that obsidian can show a moderate to very strong magnetic reaction, which it calls not a definitive test; the background is on which minerals are magnetic.
Documented negative: the scale cannot find the snowflakes. A 20.00 g piece reads 11.38 to 11.53 g in water as cristobalite and 11.42 to 11.74 g as typical obsidian. The two overlap, so a snowflake obsidian weighs like plain obsidian. The black-crystal look-alike, schorl, is separated by weight on how to tell tourmaline from obsidian; that page’s figures are not repeated here.
When the answer changes: Apache tears, basaltic glass and man-made glass
Apache tears are still obsidian. The IGS describes them as cores of unaltered glass in nodular shells of decomposed obsidian, so the core fails the definition exactly as obsidian does. Pele’s hair, the gem society’s string-like volcanic basalt glass, is glass too, of about 50% SiO2.
Natural does not mean mineral, and it does not mean old. The USGS says Newberry’s Big Obsidian Flow is the youngest volcanic feature at Newberry, covering just over 2.6 square kilometres, and part of the same eruptive event as an ash-flow tuff dated at about 1300 years. It is natural, and still not a mineral.
Man-made glass fails one clause more. The IGS warns of manufactured blue, green or red glass passed off as natural obsidian in those rare colours, and gives glass in general a specific gravity of 2.30 to 4.50 in its International Gem Society glass article. That fails the naturally occurring clause as well. General checks are on how to tell if a crystal is fake, and the organic counterpart of this question is on is amber a mineral. More definitions are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.