Basalt with Epidote/Chlorite

Physical Properties

Hardness: 5-7 (Mohs scale); Color: Dark grey to black with green patches; Luster: Dull to vitreous when wet; Structure: Fine-grained/aphanitic porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0

Formation & Geological History

Formed from rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. The green coloring suggests hydrothermal alteration where minerals like epidote or chlorite filled vesicles or replaced original matrix minerals over millions of years.

Uses & Applications

Commonly used in crushed form for construction aggregate, road base, and railroad ballast. Green-tinted specimens are sometimes used as decorative stones in landscaping or as river rock.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When basalt undergoes low-grade metamorphism, it often turns green, leading to the term 'greenstone'.

Field Identification & Locations

Identify by its heavy weight, dark base color, and fine-grained texture. The rounded shape indicates it was likely tumbled in a river or coastal environment. Found globally in volcanic regions like the Pacific Northwest, Iceland, and Hawaii.

Identified on: 4/27/2026

Mode: Standard