Obsidian
Obsidian is not a mineral, and that’s the first thing worth getting straight about it. It’s a mineraloid — specifically, volcanic glass. When lava rich in silica erupts and cools fast enough, the atoms inside it don’t have time to arrange themselves into the ordered, repeating lattice that defines a true mineral. What’s left behind is a solid with no internal crystal structure at all: no crystal system to classify it under, and no fixed chemical formula the way quartz has SiO2. Obsidian’s composition runs roughly 70 to 75% silica, but the rest varies with the parent lava, which is exactly why both fields are left null in this site’s reference data rather than filled in with an approximation.
That glassy, uncrystallized structure is also why obsidian fractures the way it does — conchoidally, in smooth, curved shells that produce edges sharp enough that obsidian blades were used for cutting tools and weapons for thousands of years before metal became widely available, and are still used today in some specialized surgical scalpels. At Mohs 5 to 5.5, it’s harder than window glass but well below quartz, and its refractive index of roughly 1.45 to 1.55 sits close to ordinary glass as well — consistent with what it actually is.
Most obsidian is black, colored by fine, evenly dispersed mineral inclusions, but the family covers real range. Snowflake obsidian gets its white, radiating patches from clusters of cristobalite crystals that grew within the glass after it solidified. Rainbow and “fire” obsidian owe their sheen to thin-film interference or aligned microscopic bubbles and inclusions that scatter light selectively, the same broad category of effect — though a different mechanism — behind labradorite’s flash. Green and brown material also occurs, depending on trace mineral content in the source lava.
Obsidian forms wherever silica-rich lava has cooled rapidly, typically at the edges of lava flows or where lava meets water. Major sources include the United States (particularly the Pacific Northwest), Mexico, Iceland, Italy, and Japan — all volcanically active regions with a long history of obsidian tool-making by the people who lived there.
Obsidian doesn’t appear on any of the historical or modern monthly birthstone lists, or on either zodiac system, tracked on this site — it’s included here as a standalone reference entry, the same way malachite is.
Gemmological properties
| Mohs hardness | 5–5.5 |
|---|---|
| Crystal system | Not recorded |
| Chemical formula | Not recorded |
| Refractive index | 1.45–1.55 |
| Color family | Black, Brown, Green, Multicolor |
| Variety of | — |
| Also known as | — |
| Origins | United States, Mexico, Iceland, Italy, Japan |
| Organic material | No |
Obsidian is a volcanic glass — a mineraloid, not a mineral. It cooled too fast for crystals to form, so it has no crystal system and no fixed chemical formula (roughly 70-75% silica, but variable), which is why both fields are null rather than missing. Snowflake and rainbow obsidian are the same glass with inclusions or thin-film effects.
Frequently asked questions
- Is obsidian a mineral?
- No. Obsidian is a mineraloid — a naturally occurring solid that lacks the ordered crystal structure that defines a true mineral. It's volcanic glass, formed when lava rich in silica cools too fast for crystals to grow, so it never develops a crystal system at all.
- Why doesn't obsidian have a fixed chemical formula?
- Because it's glass, not a crystalline compound. Its composition is roughly 70-75% silica, but the remainder varies with whatever lava it formed from, so there's no single formula the way there is for a mineral like quartz (SiO2). That variability is also why obsidian's exact color and clarity differ from flow to flow.
- Is obsidian a birthstone on any of the lists this site tracks?
- No. It doesn't appear on any of the historical, US, UK, or zodiac systems documented on this site — it's covered here as a standalone gemstone reference entry.