Fire Obsidian Identification Guide

What Fire Obsidian Looks Like

Fire obsidian is a rare variety of volcanic glass (obsidian) that displays an intense, fiery iridescent sheen — bands of gold, red, green, blue, and violet. The effect is caused by nanometer-thin layers of magnetite nanocrystals within the glass, which act as a natural thin-film reflector. On a dark glassy base, polished fire obsidian flashes vivid spectral color.

• Color: black glass base with brilliant rainbow iridescent sheen
• Luster: vitreous (glassy)
• Transparency: opaque to translucent on thin edges
• Form: massive glass; the fire appears on properly oriented polished surfaces

Step-by-Step Field ID Checklist

1. Confirm a glassy black base — obsidian has no crystals and a smooth glassy feel.
2. Tilt the polished surface to see intense, layered rainbow fire that shifts with angle.
3. Check for conchoidal fracture — curved, shell-like breaks with razor-sharp edges.
4. Test hardness — about 5–5.5, so it will scratch with a steel file.
5. Note the orientation dependence — the fire appears only where the magnetite layers are properly exposed.

Key Diagnostic Tests

• Mohs hardness 5–5.5: Softer than quartz; a steel knife or quartz crystal will scratch it. This separates it from fire agate (6.5–7).
• Fracture: Pronounced conchoidal fracture with sharp edges — classic volcanic glass.
• Streak: White.
• Density: ~2.4 g/cm³, slightly lighter than quartz.
• Magnetism: May show very faint magnetic response from the magnetite layers, though usually too weak to detect with a hand magnet.
• No crystallinity: Being glass, it shows no cleavage and no mineral grains.

Common Look-Alikes and How to Tell Them Apart

• Rainbow obsidian: Also obsidian with sheen from magnetite, but the color is typically a softer, deeper single- or double-color sheen rather than the vivid multicolor fire bands; fire obsidian shows brighter, sharper spectral layering. Both are the same material with differing layer geometry.
• Fire agate: Harder (Mohs 6.5–7), brown botryoidal chalcedony with internal fire; fire obsidian is glassy black with surface-oriented sheen and softer.
• Labradorite: A feldspar with labradorescence; it has cleavage and Mohs ~6–6.5, while obsidian is glass with conchoidal fracture and no cleavage.
• Dyed or coated glass imitations: Manufactured glass lacks the natural magnetite layering; the sheen looks painted-on or uniform rather than depth-oriented.

Where It Is Typically Found

Gem-quality fire obsidian is famously associated with Glass Buttes, Oregon (USA), the premier source, where rhyolitic obsidian flows contain the right magnetite-layered material. Obsidian in general forms wherever silica-rich (rhyolitic) lava cools too quickly to crystallize, so it is found around young felsic volcanoes worldwide, but the specific fire variety is uncommon and prized.