igneous
Porphyritic Basalt or Letter Stone
Porphyritic Basalt with Plagioclase Phenocrysts
AI-generated identification · may be incorrectHardness of 6 on the Mohs scale. Features distinct white or pale lath-shaped feldspar crystals set in a fine-grained, dark brown to grayish-black groundmass.
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Physical properties
Hardness of 6 on the Mohs scale. Features distinct white or pale lath-shaped feldspar crystals set in a fine-grained, dark brown to grayish-black groundmass.
Formation & geological history
Formed through a two-stage cooling process in volcanic settings, where initial slow cooling produced large crystals followed by rapid cooling of the surrounding matrix.
Uses & applications
Used primarily as decorative aggregate, educational geological specimens, and occasionally in lapidary work for cabochons.
Geological facts
The distinct crystal patterns often resemble ancient runes or letters, earning this variety the historical moniker of letter stone.
Field identification & locations
Easily identified in the field by contrasting white or greenish rectangular phenocrysts embedded in a dark volcanic background.
