Fireworks Obsidian Identification Guide

What Fireworks Obsidian Looks Like

Fireworks obsidian is a variety of black volcanic glass containing radiating clusters of cristobalite spherulites — feldspar/silica crystallites that grew outward in starburst sprays as the glass began to devitrify. Against the glossy black base, these whitish-grey radiating bursts look like exploding fireworks, hence the name. It is closely related to snowflake obsidian but with a more explosive, spraying pattern.

• Color: black to dark grey glass with white/grey radiating bursts
• Luster: vitreous (glassy)
• Transparency: opaque to translucent on thin edges
• Form: massive glass; spherulites are radial fibrous aggregates

Step-by-Step Field ID Checklist

1. Confirm a glassy black base — smooth, glossy, no visible mineral grains.
2. Look for radiating starburst clusters of pale crystallites that spray outward from central points.
3. Check for conchoidal fracture — curved shell-like breaks with sharp edges.
4. Test hardness — about 5–5.5, scratched by a steel file.
5. Distinguish bursts from the blocky blobs of ordinary snowflake obsidian.

Key Diagnostic Tests

• Mohs hardness 5–5.5: Volcanic glass; a steel knife and quartz both scratch it. Softer than the quartz-family stones.
• Fracture: Strongly conchoidal with razor-sharp edges — the classic obsidian break.
• Streak: White.
• Density: ~2.4 g/cm³.
• Texture: The radiating spherulites are the diagnostic feature — they are radial fibrous growths of cristobalite/feldspar in the glass.
• No cleavage: Being glass, it has none.

Common Look-Alikes and How to Tell Them Apart

• Snowflake obsidian: The closest relative — same material, but its spherulites form rounded, snowflake-like blotches rather than sharp radiating bursts. The starburst geometry distinguishes fireworks obsidian.
• Flake / leopard obsidian: Has scattered crystallite flecks rather than organized radiating sprays.
• Porphyritic obsidian: Contains discrete feldspar phenocrysts, not radial spherulite clusters.
• Granite/diorite: Crystalline igneous rocks with interlocking visible crystals; fireworks obsidian is glass with conchoidal fracture and no true crystal framework.

Where It Is Typically Found

Fireworks and snowflake obsidian form where rhyolitic lava cooled rapidly to glass and then partially devitrified, allowing spherulites to nucleate and grow. Major sources include the western United States (Oregon, Nevada, Utah, Arizona, California) and other young felsic volcanic regions worldwide such as Mexico and parts of the Mediterranean. Look for it in obsidian flows and in weathered float around rhyolite domes.