How to tell tourmaline from obsidian
Black tourmaline is a crystal, density 3.18 to 3.22; obsidian is volcanic glass, usually 2.33 to 2.42. A 20 g piece reads 13.71 g against 11.74 g in water.
Weigh it, then read it. Black tourmaline (schorl) is a crystal, hardness 7, density 3.18 to 3.22 on its Handbook sheet. Obsidian is volcanic glass, not a mineral: hardness 5, typically 2.33 to 2.42 (IGS). A 20.00 g piece reads 13.71 to 13.79 g in water as schorl, 11.42 to 11.74 g as obsidian. A refractometer confirms it.
In short
- One is a mineral and one is a glass. Schorl, the black tourmaline, is a crystalline borosilicate with hardness 7 on its Handbook of Mineralogy sheet. The USGS describes obsidian as a natural glass with no crystalline structure, and the International Gem Society calls it a rock, hardness 5.
- The 3.00 line: no obsidian in the gem society's range is denser than 3.00, and no schorl on its Handbook sheet is lighter than 3.18. A 20.00 g piece reads 13.71 to 13.79 g in water as schorl and 11.42 to 11.74 g as typical obsidian. Even the densest basaltic glass, at 3.00, reads 13.33 g, 0.38 g short of schorl.
- The sources disagree, and the page shows it. The IGS says schorl's specific gravity overlaps obsidian's, because its own schorl range runs from 2.82 to 3.24. The Handbook's schorl, 3.18 to 3.22, does not overlap. Typical obsidian, 2.33 to 2.42, is clear of both ranges, so for an ordinary black piece the scale decides either way.
- The refractometer settles the rest. Obsidian is isotropic with one index, 1.48 to 1.51, usually 1.49, on the gem society's figures. Schorl's indices are 1.635 to 1.671 on its sheet, a gap of at least 0.125. The IGS says an RI test should separate them.
- Shape is the free first look. Schorl grows as prisms striated along their length, with a trigonal outline. Obsidian is amorphous and is, in the IGS's words, the best example of conchoidal fracture. Schorl's fracture is uneven to conchoidal too, so a curved break alone decides nothing.
| Criterion | Black tourmaline (schorl) | Obsidian | Does it choose? |
|---|---|---|---|
| What it is | A mineral of the tourmaline group | A natural glass with no crystalline structure (USGS); a rock (IGS) | - |
| Form | Prisms to 1.5 m, trigonal, striated parallel to [0001]; also radial, fibrous, massive | Amorphous: rounded masses, broken pieces, flows (IGS) | Yes, when faces show |
| Hardness (Mohs) | 7 | 5; 6 for basalt glass (IGS) | Yes, but it marks the piece |
| Density / specific gravity | 3.18 to 3.22 (Handbook); 2.82 to 3.24 (IGS) | 2.25 to 3.00; typically 2.33 to 2.42 (IGS) | Yes for typical obsidian |
| 20.00 g piece, weight in water | 13.71 to 13.79 g | 11.42 to 11.74 g typical; 13.33 g at most | Yes: about 2 g apart for typical obsidian |
| Refractive index | Two: ω 1.660 to 1.671, ε 1.635 to 1.650 | One: 1.48 to 1.51, usually 1.49 | Yes: at least 0.125 apart |
| Pleochroism | Very strong (sheet) | None (IGS) | In thin or transparent pieces |
| Fracture | Uneven to conchoidal | Conchoidal (IGS) | No |
| Cleavage | Very poor | None (IGS) | No |
| Lustre | Vitreous to resinous | Vitreous (IGS) | No |
| Inclusions under 10x | Hollow tubes, typically parallel to the length (IGS) | Round, torpedo or teardrop bubbles, often in parallel; white cristobalite spherulites in snowflake obsidian (IGS) | Often |
How to tell tourmaline from obsidian: crystal or glass?
Black tourmaline is a crystal and obsidian is a glass, so the first question is whether the piece has any crystal shape at all. The Handbook of Mineralogy sheet for schorl describes prisms with a prominent trigonal prism and pyramid, commonly hemimorphic and striated parallel to the long axis, to 1.5 m. The USGS describes obsidian as a natural glass with no crystalline structure, and the International Gem Society describes it as amorphous, usually as rounded masses, broken pieces, filaments or flows.
Obsidian is not a mineral, and the difference is the point. A mineral has an ordered atomic arrangement; a glass does not. Obsidian has no row on the IMA list, which is set out on crystal vs mineral: what is the difference. The IGS gives obsidian about 66 to 72% SiO2, against 34.22% worked for ideal schorl on is tourmaline a quartz.
Where shape fails. Tumbled stones, beads and carvings have no faces, and both materials can break with a curved, shell-like fracture: the schorl sheet records uneven to conchoidal, and the IGS calls obsidian the best example of conchoidal fracture. Then the scale decides. The identification pages are indexed at the collecting guides.
Weigh it: the 3.00 line, worked
The 3.00 line: obsidian, on the International Gem Society's figures, never exceeds specific gravity 3.00, and schorl, on its Handbook sheet, is never below 3.18. Weight in water is mass times one minus one over density. For a 20.00 g piece that gives 13.71 to 13.79 g for schorl and 11.42 to 11.74 g for typical obsidian at 2.33 to 2.42. Basaltic glass, the IGS's densest obsidian at 2.70 to 3.00, reads 12.59 to 13.33 g, still 0.38 g below schorl. In the hand, a 2 cm cube is 25.44 to 25.76 g of schorl against 18.64 to 19.36 g of typical obsidian.
A documented disagreement. The IGS obsidian article says the specific gravity range of schorl overlaps with that of obsidian. That follows from its own tourmaline table, which gives schorl 2.82 to 3.24; the Handbook prints a measured 3.18 to 3.22 and a calculated 3.244. We print both. On the IGS figures a schorl at the very bottom of its range would read 12.91 g, inside basaltic glass. On either reading, an ordinary black obsidian at 11.42 to 11.74 g is clear of every schorl figure by more than 1 g.
So use the scale for the common case and the refractometer for the edge. A reading near 11.5 g on a 20 g piece is obsidian; a reading near 13.7 g is schorl on the Handbook figure; a reading between 12.6 and 13.4 g is either dense glass or light tourmaline and needs the next test. The method is on how to measure specific gravity at home.
The refractometer: one reading near 1.49, or two above 1.63
Obsidian gives one refractive index, 1.48 to 1.51 and usually 1.49; schorl gives two, between 1.635 and 1.671. The International Gem Society lists obsidian as isotropic, with no birefringence apart from crystal inclusions, and says an RI test should separate it from schorl. The Handbook of Mineralogy sheet for schorl gives ω 1.660 to 1.671 and ε 1.635 to 1.650. The smallest gap between them is 0.125.
A dark schorl may show only one line. The IGS notes that tourmaline can absorb one ray totally, giving a single line on a refractometer. That line still sits above 1.62, nowhere near obsidian's 1.49, so position decides even when the gap cannot be seen. The full threshold table is on how to tell if tourmaline is real.
Other black look-alikes. The IGS names jet, chalcedony and schorl as natural materials confused with obsidian, and notes manufactured blue, green or red glass passed off as rare obsidian colours. Black glass behaves like obsidian on the refractometer but often carries round bubbles and swirl marks.
Snowflake, sheen and Apache tears: what the names mean
All three are obsidian, named for what is in the glass or how it weathers. The International Gem Society describes snowflake obsidian as glass with snowflake-like spherulites of cristobalite, sheen obsidian as glass with inclusions of gas bubbles, and Apache tears as cores of unaltered glass in nodular shells of decomposed obsidian. None of them is tourmaline.
Inclusions tell them apart under a lens, in our judgement more quickly than any instrument. The IGS lists obsidian's bubbles as elongated, torpedo-shaped, round or teardrop-shaped, often in parallel. It lists tourmaline's as hollow tubes, typically running parallel to the length. A black prism with parallel tubes and a white streak is the schorl pattern; the schorl sheet records the streak as white, and the method is on how to use a streak plate.
Labels, not verdicts. A piece sold as black tourmaline that weighs like obsidian deserves the refractometer before it is relabelled. Nothing on this site is offered for sale; the wanted list is the only commercial route.