Pitchstone
Hydrated volcanic glass (variable silicate)
A dull, resinous volcanic glass similar to obsidian but with higher water content and a waxy pitch-like luster.
- Mohs hardness
- 5-6
- Color
- Black, brown, green, grey to mottled
- Type
- igneous
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Overview
Pitchstone is a volcanic glass closely related to obsidian but distinguished by its dull, waxy, resin-like (pitchy) luster rather than obsidian's bright, glossy shine. The dull appearance comes from a higher water content and abundant tiny crystallites scattered through the glass.
It occurs in black, brown, green, grey, and mottled colors, and can have a granular or porphyritic texture where small crystals (phenocrysts) sit in the glassy matrix. Its water content typically ranges from about 4% to 10%, much higher than obsidian.
Pitchstone forms where viscous, silica-rich lava or shallow intrusions cool quickly enough to quench into glass.
Formation & geology
Pitchstone forms when silica-rich, water-bearing magma cools rapidly, quenching to glass before crystals can fully grow. The higher water content and numerous microscopic crystallites give it a duller luster than ordinary obsidian.
It typically occurs in lava flows, the chilled margins of dikes and sills, and shallow intrusions. Notable localities include the Isle of Arran and Eigg in Scotland, the Meissen area of Germany (the original 'Pechstein'), and various rhyolitic volcanic provinces worldwide.
How to identify it
Look for a dark, glassy rock with a dull, greasy, resin- or pitch-like luster rather than the bright glassy shine of obsidian. It often shows conchoidal fracture and may contain small visible crystals.
The waxy, dull luster is the key distinction from obsidian, which is glossy and vitreous. Pitchstone is also generally more crystallite-rich and may feel slightly rougher.
Look-alikes include obsidian (brighter luster, lower water), pitch/asphalt (organic, much softer), and dark chert (no glassy fracture sheen). A conchoidal break with a resinous sheen confirms pitchstone.
Uses & significance
Pitchstone has been used since prehistoric times as a material for stone tools and flakes, much like obsidian, because it fractures to sharp edges; Scottish pitchstone artifacts are well documented in archaeology.
Today it is mainly of interest to collectors, geologists, and lapidaries, who may cut and polish attractive specimens into cabochons or ornamental pieces. It has been used as a decorative stone. Metaphysically it is sometimes grouped with obsidian as a grounding stone, though this is not a mineralogical property.
Frequently asked questions
What is the difference between pitchstone and obsidian?
Both are volcanic glass, but pitchstone has a dull, resinous luster and higher water content, while obsidian is bright and glossy with less water.
Why does pitchstone look dull?
Its higher water content and abundant microscopic crystallites scatter light, giving it a waxy, pitch-like sheen instead of a glassy shine.
Was pitchstone used for tools?
Yes. Like obsidian, it fractures to sharp edges and was used in prehistory for blades and flakes, notably in Scotland.
What colors does pitchstone come in?
It occurs in black, brown, green, grey, and mottled varieties depending on composition and included crystals.
Pitchstone guides
In-depth guides for identifying, valuing, and understanding Pitchstone.
Pitchstone identified by the community
Real specimens identified with Rock Identifier.
