A black rock in a drawer. It breaks with a smooth curved shell, like a chip of glass, and it sits in the hand heavier than it looks. Two very different stones do this, and people mix them up constantly: obsidian and chert.
They are not related. Obsidian is glass. Lava cooled so fast that no crystal had time to form, the atoms locked where they fell. Its black is not a mineral you can see: it is microscopic crystals of iron oxide and magnetite scattered through the glass in numbers large enough to swallow the light. Chert is the opposite mistake. It is all crystal, quartz, grown so fine you cannot see the grains at any magnification your eye can reach. Its black comes from organic carbon and sulfides trapped while it hardened in sediment, usually on a sea floor.
Both break the same way, and that is why one word gets used for both. Glass and microcrystalline quartz have no planes of weakness, so a crack has to find its own path, and that path curves. Conchoidal fracture. The shell-shaped chip is the tell that says “not granite, not sandstone” and nothing more.
The test that separates them costs nothing: light.
Take the thinnest edge, or a flake if you have one, and hold it against a lamp. Obsidian, even a black one, passes colour through a couple of millimetres of edge: amber, olive, brown, sometimes a grey-green. It looks like light going through glass, because it is. Chert is stingy. A true flake edge may pass a dull grey light, and that is all; a cobble of black chert shows you nothing at the thickness of the stone. If the edge lights up like a bottled sunset, you have obsidian. If it swallows the light, you have chert.
Second test, and it does something the flashlight cannot: hardness. Chert is quartz, 7 on the scale, and it scratches window glass, a steel knife, and most other stone. Obsidian sits near 5.5, about the same as that window glass, so it scratches neither. Run a knife edge across it and you feel the blade bite instead of skate.
Where each one comes from is the real difference, and it is a difference in time.
Obsidian does not last. Glass wants to be crystal, and given water and a few million years it gets its wish: water diffuses in and the atoms begin to organize, a process called devitrification. Almost no obsidian on Earth is older than about twenty million years, which is young for a rock. The white patches in snowflake obsidian are exactly that process, started: clusters of crystals growing inside the glass.
The watery rind that forms on a broken surface thickens at a measurable rate, and archaeologists use it as a clock. Obsidian hydration dating, put to work by Irving Friedman and Robert Smith in 1960. A blade struck five thousand years ago carries a rind that a blade struck last week does not.
It is also the sharpest thing we know how to make. A freshly struck obsidian edge tapers to roughly three nanometres, an order of magnitude finer than a steel scalpel will hold. Surgical blades are cut from it for exactly that reason.
If your stone turns out to be chert, the story is quieter and much older: silica dissolved from sponge spicules and plankton, gelling and hardening inside sediment over tens of millions of years. That is why a black chert cobble in a Tennessee creek carries a dead sea inside it. It set like a gel, and then it was squeezed into stone.
Two black rocks. One is a young fire that cooled too fast and is slowly giving in. The other is a very old sea, crystallized past the point where the eye can follow.
Both are worth naming. If you have one and cannot place it, send photos, dry in daylight and then wet, and tell me where you found it. My first read is free. A full specimen read is $20 by card, and it includes the tests to run yourself.
[email protected] or paper.wf/minty-stones
#obsidian #chert #rocks #minerals #rockhounding
