Pitchstone Identification Guide

What Pitchstone Looks Like

Pitchstone is a volcanic glass similar to obsidian but with a higher water content (typically several percent), giving it a dull, resinous, "pitch"-like (tar-like) luster rather than the bright glassy shine of obsidian. It often contains microscopic crystallites and visible phenocrysts.

• Color: Dark grey, green, brown, black, or reddish, often mottled or streaky.
• Luster: Resinous to waxy, dull — like hardened pitch or resin.
• Transparency: Opaque to slightly translucent on edges.
• Texture: Glassy groundmass, frequently with small crystals (phenocrysts) and flow banding.

Step-by-Step Field ID Checklist

1. Check the luster. A dull, resinous, pitchy sheen (not bright glassy) is the key distinction from obsidian.
2. Look for conchoidal fracture with curved, shell-like surfaces.
3. Use a loupe for tiny embedded crystals and flow lines.
4. Note the mottled, streaky color.
5. Hardness test — scratches with difficulty around 5–5.5.

Key Diagnostic Tests

• Mohs hardness: ~5–5.5.
• Streak: White to pale grey.
• Fracture: Conchoidal; no cleavage.
• Density: ~2.3–2.5 g/cm³ — light, like other volcanic glass.
• Other: Higher water content than obsidian; tends to be more crystalline (devitrified) internally.

Common Look-Alikes and How to Tell Them Apart

• Obsidian: Bright, vitreous luster and usually fewer crystallites; pitchstone is duller and resinous with more inclusions. The dull pitch-like sheen is the main field separator.
• Basalt: Crystalline and granular (not glassy), no conchoidal glass fracture.
• Tachylite (basaltic glass): Also glassy but typically black; chemistry and locality help, and pitchstone is usually more silicic.
• Jet/lignite: Much lighter (lower density), can be warm to the touch, burns/smells when hot-pointed; pitchstone is mineral glass.
• Resin/amber: Far softer (2–2.5), much lighter, warm feel, floats in saltwater.

Where Pitchstone Is Found

Pitchstone occurs in silicic lava flows and shallow intrusions (dykes and sills). Classic localities include the Isle of Arran and other parts of Scotland, plus volcanic terranes in Iceland, New Zealand, and the western USA.