Obsidian vs Black Tourmaline: Volcanic Glass or Striated Crystal?

Short answer: obsidian is natural volcanic glass with no crystal structure; black tourmaline is schorl, a boron silicate that grows as long, grooved crystals. Raw pieces are easy: schorl shows parallel striations and a rounded triangular cross section, obsidian shows smooth, shell-shaped breaks. Polished pieces take two quick checks. Schorl is far heavier for its size (specific gravity about 3.15 against about 2.4) and harder (7 to 7.5 against about 5.5), and its glassy shine looks faintly metallic.

Why is one a glass and the other a crystal?

That single difference drives every test on this page. Geology.com explains that obsidian forms when molten rock cools so rapidly that atoms are unable to arrange themselves into a crystalline structure, leaving an amorphous material it classes as a mineraloid (Geology.com, obsidian). No crystal structure means no crystal faces, no cleavage and no preferred direction: obsidian breaks the same way in every direction, with curved, rippled fractures and very sharp edges.

Schorl is the opposite. It is a true mineral with the formula NaFe3Al6(BO3)3Si6O18(OH)4, and Minerals.net notes that schorl is best known as "black tourmaline" and is the most common form of tourmaline (Minerals.net, schorl). It grows in prisms that preserve a record of their growth: grooves along their length and a cross section that GIA calls unique among common minerals, noting that no other common mineral has three-sided prisms (GIA, tourmaline).

So the question to ask of any black stone is simple: does it have a grain?

Raw pieces: start with shape

Look along the length

Schorl crystals are usually elongated and heavily striated, with deep grooves running parallel to the long axis. Many have a rounded, triangular cross section and a flattened pyramidal tip, according to Minerals.net. Even a broken chunk of schorl tends to show these parallel ridges on at least one side, like a bundle of pencils fused together.

Obsidian has no grooves, only conchoidal fracture: smooth, curved surfaces with concentric ripple marks. A raw obsidian chunk looks like broken bottle glass, because in a real sense it is.

Check where it came from

Context helps. Schorl is found most often in granite pegmatites, and Geology.com notes that black schorl is the variety most often found as scattered accessory crystals in granite, pegmatite and gneiss (Geology.com, tourmaline). A black prism sitting in white feldspar or quartz is schorl. A black lump with a gray, frosty rind from a volcanic field is obsidian; small nodules of this kind are sold as Apache tears.

What still works once a piece is tumbled or carved?

Once a stone is tumbled into a smooth pebble or turned into a sphere, shape is gone. These checks remain.

1. Heft it against something you know

This is the fastest test. Schorl's density averages 3.15, per Webmineral, while the International Gem Society gives obsidian's typical specific gravity as 2.33 to 2.42 (International Gem Society, obsidian). Two stones of the same size differ by roughly a third in weight. A palm stone of black tourmaline feels like a lump of metal next to an obsidian one. With a kitchen scale and a cup of water, the specific gravity test gives you a number in a couple of minutes.

2. Look for the grain under a loupe

Tumbling rarely erases every trace of schorl's structure. With a 10x loupe and a raking light, look for faint parallel lines, tiny fractures running the same way, or a slightly fibrous texture at the edges of pits. Polished obsidian shows a flawless, featureless surface, sometimes with aligned gas bubbles or bands, which the IGS lists among its typical inclusions, but never a crystal grain.

3. Hold a thin edge to a strong light

Minerals.net says schorl is exclusively black and is never transparent or even translucent. Obsidian, being glass, often lets a smoky brown or gray glow through its thinnest edges when held against a flashlight, and the IGS notes that obsidian ranges from transparent to opaque. Light through the edge points to obsidian. A dead black edge is consistent with schorl, but not proof, since some obsidian is opaque.

4. Hardness, used carefully

Geology.com puts obsidian at about 5.5 on the Mohs scale; schorl is 7 to 7.5. A steel knife (5 to 6.5 on Geology.com's Mohs table) can mark obsidian but will not touch schorl. Try it only on an unpolished spot, because a scratch on a finished sphere is permanent.

The two side by side

Sources: Geology.com, Minerals.net, Webmineral and the International Gem Society.

That refractive index row is how a gemologist settles cut stones. The IGS lists jet, chalcedony and schorl among the natural stones that get substituted for or confused with obsidian, and says that while the specific gravity ranges can overlap in unusual material, an RI test separates them. Schorl's high birefringence (0.027 to 0.032 on Webmineral) also shows up as doubled facet edges under magnification; obsidian, as glass, shows none.

Where does each one come from?

Obsidian only survives in young volcanic terrain. Geology.com notes that obsidian older than a few million years is rare because the glass is destroyed or altered by weathering and heat, and that in the United States it is not found east of the Mississippi River. It lists significant deposits in Argentina, Chile, Ecuador, Greece, Iceland, Italy, Japan, Kenya, Mexico, New Zealand and many other countries, and in the western US across Oregon, California, Arizona, Nevada and neighboring states. Geology.com also notes that most obsidian used in the jewelry trade is produced in the United States.

Schorl is a granite and pegmatite mineral. Minerals.net names outstanding crystals from the Shigar Valley and Stak Nala in Pakistan's Skardu District, the Erongo region of Namibia, Minas Gerais in Brazil, and in the US from San Diego County, California and from Maine, Connecticut and New Hampshire. Collector-grade schorl usually comes as sharp crystals on white albite, which is why matrix pieces command more than loose rough.

Which one is more likely to be faked?

Obsidian. Because it is glass, glass imitates it well. The IGS warns that artificial blue, green or red glass is passed off as rare-colored obsidian, and notes that natural obsidian may hold crystallites, straight banding or needles that factory glass lacks. It adds a lesser-known check: although not definitive, a magnetic reaction can help confirm a piece as natural, since obsidians can react moderately to very strongly to a strong magnet. See the glass imitations guide for the wider picture.

Black tourmaline is cheap and abundant, so it is rarely imitated outright. The realistic risk is the reverse: a tumbled black stone of some other kind sold as tourmaline. The weight and loupe tests above catch most of those.

Price, wear and which to buy

Both are inexpensive in tumbled and rough form. Prices climb along different lines. For obsidian, pattern and phenomenon drive value: a crisp snowflake pattern, which Geology.com attributes to radial clusters of white or gray cristobalite, or a strong rainbow or silver sheen. For schorl, crystal quality drives value: sharp terminations, bright luster, and contrast against an albite or quartz matrix.

Durability favors tourmaline. Obsidian lacks toughness and chips easily, so Geology.com recommends it for earrings and pendants rather than rings and bracelets. Schorl is harder but brittle along its length.

• For a carved figure, sphere or mirror-polished piece: obsidian. Glass carves cleanly and takes a liquid shine, and the sheen varieties only exist in obsidian.
• For a natural specimen with visible structure: schorl, ideally a terminated crystal or a piece on matrix.
• For a pocket stone that gets handled every day: tumbled schorl tolerates knocks a little better.

In crystal traditions both are counted among the "protective" black stones; that shared association is part of why they get confused. Our obsidian guide and tourmaline guide cover each stone in depth, and the planned black stone identification guide widens the field to onyx, jet and hematite.

What to look for when you buy obsidian or black tourmaline

For raw schorl, the photos should show striations and ideally a termination or a natural crystal face; a listing weight lets you sanity-check density against the size. For obsidian, look for an even polish with no orange-peel texture, and on sheen pieces ask for a photo under a single light, because sheen only shows at one angle.

• Black Tourmaline Crystal Raw Hunk, elongated specimen: 588 grams with the columnar, striated structure running its full length, the textbook schorl look.
• Black Tourmaline Tumbles: small polished schorl for practicing the heft and loupe tests on stones that have lost their shape.
• Black Obsidian Crystal Sphere, polished volcanic glass: a mirror-polished sphere that shows obsidian's featureless, glassy surface.
• Mahogany Obsidian: black and brown swirled glass, a pattern schorl never produces.
• Silver Obsidian Palmstones: sheen obsidian; tilt it under one bulb to see the silver flash that rules out tourmaline at a glance.

Frequently asked questions

Is black obsidian the same as black tourmaline?

No. Obsidian is volcanic glass that cooled too fast to crystallize. Black tourmaline is schorl, a crystalline boron silicate that grows in striated prisms in granite and pegmatite. They differ in hardness, density, structure and optics, and they form in completely different geological settings.

Which is heavier, obsidian or black tourmaline?

Black tourmaline, by about a third for the same size. Schorl has a specific gravity around 3.15, while typical obsidian runs 2.33 to 2.42. Two palm stones of equal size make the difference obvious in the hand, and a kitchen scale with a cup of water will confirm it.

Why does my black tourmaline have lines on it?

Those parallel grooves are striations, growth features that run along the length of tourmaline crystals. They are one of the best identifiers for schorl. Obsidian, being glass, never has striations; it breaks into smooth, curved surfaces instead.

Can obsidian be fake?

Yes. Manufactured glass is sometimes sold as obsidian, especially in unusual colors such as blue, green or red. Natural obsidian may show crystallites, straight banding or needle inclusions under magnification, and many obsidians react to a strong magnet. Black tourmaline is so cheap that outright fakes are uncommon.

Is black tourmaline magnetic?

Schorl contains iron, but magnetism is not a reliable way to separate it from obsidian, which can also react. Use weight, visible striations and hardness instead. The more distinctive electrical trait of tourmaline is that it becomes charged when heated or squeezed.

Sources

• Geology.com, Obsidian: https://geology.com/rocks/obsidian.shtml
• Geology.com, Tourmaline: https://geology.com/minerals/tourmaline.shtml
• Geology.com, Mohs Hardness Scale: https://geology.com/minerals/mohs-hardness-scale.shtml
• Minerals.net, Schorl: https://www.minerals.net/mineral/schorl.aspx
• Webmineral, Schorl Mineral Data: http://webmineral.com/data/Schorl.shtml
• International Gem Society, Obsidian Value, Price, and Jewelry Information: https://www.gemsociety.org/article/obsidian-jewelry-and-gemstone-information/
• GIA, Tourmaline: https://www.gia.edu/tourmaline