Porphyritic Andesite

Hardness: 6-7, Color: Gray with dark spots, Luster: Dull, Crystal structure: Porphyritic (large crystals in fine matrix)

Formed from the rapid cooling of lava at or near the Earth's surface, often associated with subduction zones. The dark spots are phenocrysts (larger crystals) that formed before the magma erupted and cooled quickly.

Often used in construction and as road base. Polished pieces can be used for decorative purposes or in geology collections.

Andesite is named after the Andes Mountains, where it is very common due to subduction zone volcanism.

Identify by its gray color and porphyritic texture (dark spots in a lighter gray matrix). Commonly found in volcanic regions. Tips for collectors: Look for varied phenocryst sizes to understand cooling history.