Porphyritic Andesite or Basalt

Greenish-grey color, porphyritic texture with visible phenocrysts in a fine-grained groundmass, hardness of 6 on Mohs scale, dull luster on weathered surfaces.

Formed by volcanic processes. The two distinct crystal sizes indicate a two-stage cooling history: slow cooling deep underground to form the larger phenocrysts, followed by rapid cooling upon eruption or shallow intrusion to form the fine-grained matrix.

Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast.

The greenish color may be due to low-grade metamorphism or alteration (e.g., chloritization, epidotization).

Identify by its porphyritic texture (spots in a solid background). Common in volcanic arcs and continental margins.