How Is Obsidian Formed?

Obsidian: volcanic glass, not crystal

Unlike most stones, obsidian isn't a crystal—it's natural volcanic glass. It's classified as a mineraloid because its matter is amorphous: atoms aren't arranged in an orderly network like in quartz, but frozen in disorder, exactly like window glass.

It's composed of more than 70% silica (SiO₂), plus traces of aluminum, sodium, and potassium. Its composition is close to granite or rhyolite; what changes everything is its glassy texture, born from a particular cooling process.

How obsidian forms

It all begins with an eruption of felsic lava (called "acidic"), extremely rich in silica. This lava is tremendously viscous: it flows slowly, like thick honey. Inside, long silica chains intertwine and prevent atoms from circulating.

But for minerals to form crystals, they need time and mobility. Here, neither exists: the lava solidifies before crystals can form. Result: a dense, homogeneous glass with no crystal structure—obsidian.

For decades, scientists explained this as "flash cooling." Recent research nuances that story: forming such dense glass without bubbles actually requires slower cooling than once thought, over several weeks, allowing gas bubbles to escape. One thing remains certain: it's the absence of crystallization that makes all the difference.

Why obsidian is black

Its dark color comes mostly from small amounts of iron and magnesium scattered through the glass. Depending on impurities and inclusions trapped during the flow, obsidian can show other appearances:

• white spots (cristobalite crystals) → snowflake obsidian;
• aligned gas bubbles creating a golden or silver sheen → sheen obsidian;
• magnetite nanoparticles that decompose light → rainbow obsidian and celestial eye.

I detail all these varieties in my complete obsidian guide.

Where obsidian is found

Obsidian forms wherever there's been silica-rich volcanic eruptions, often at the edges and surface of rhyolite flows or on the margins of volcanic domes. On these edges, cooled faster and cleaner, the finest material is found.

Famous deposits include Mexico, the United States (Glass Butte and Big Obsidian Flow in Oregon, Obsidian Cliff in Yellowstone), Iceland (Mount Hekla), Italy (Aeolian Islands, especially Lipari), but also Armenia, Turkey, Japan, and Greece. All volcanic lands.

A stone that ages

Geological curiosity: obsidian is metastable. In other words, this glass isn't eternal: very slowly, over millions of years, it reorganizes into fine crystallizations—a process called devitrification, which creates the snowflake patterns.

That's why you find almost no very ancient obsidian: the oldest deposits have already transformed. Water speeds this process: in contact with groundwater, obsidian hydrates gradually and turns into perlite, a duller glass. The black, brilliant stone you wear is therefore, on Earth's timescale, quite young.

From raw rock to jewelry

The same material that once made blades and arrowheads—thanks to its conchoidal fracture with razor-sharp edges—becomes jewelry today. The work involves choosing beautiful shards, cutting them carefully, then polishing until reaching that deep black, almost liquid appearance.

With me, each stone is selected one by one, then assembled by hand. That attention is what makes every piece unique. You can see my obsidian bracelets or browse the full shop.

And if you worry about glass imitations, it's fair: I explain how to spot real obsidian in my guide.

Frequently asked questions