igneous
Porphyritic Basalt (likely artificially dyed or altered)
Basalt with plagioclase phenocrysts (likely treated)
Hardness: 6 (groundmass) to 6.5 (phenocrysts). Color: Natural dark grey/black groundmass with white lath-like crystals; this specimen shows an unusual vivid blue tint. Luster: Dull to sub-vitreous. Texture: Porphyritic (large crystals in a fine-grained matrix).
- Hardness
- 6 (groundmass) to 6
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6 (groundmass) to 6.5 (phenocrysts). Color: Natural dark grey/black groundmass with white lath-like crystals; this specimen shows an unusual vivid blue tint. Luster: Dull to sub-vitreous. Texture: Porphyritic (large crystals in a fine-grained matrix).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The large white phenocrysts (plagioclase feldspar) began crystallizing underground before the lava erupted and the rest of the rock cooled quickly. Found in volcanic regions globally.
Uses & applications
Commonly used as crushed stone for construction, road base, and railroad ballast. Distinctive porphyritic specimens are often kept by collectors for educational purposes.
Geological facts
The regional coordinates provided (Saint Petersburg, Russia) are in a sedimentary basin (Russian Platform), but the region contains significant glacial 'erratics' transported by ice from the Fennoscandian Shield to the north, where volcanic rocks are common. However, the vivid blue color is highly atypical for natural basalt and suggests the rock may have been dyed or found near industrial waste.
Field identification & locations
Identify by the contrast between white, rectangular feldspar crystals and the darker fine-grained background. In the field, look for volcanic outcrops or glacial till. The blue hue remains suspicious and is likely not a natural mineral stain.
