Basalt cobble

Dark gray-black, fine-grained, matte to faintly vitreous, with small vesicles and rounded water-worn surfaces; hardness about 5–6 Mohs, no obvious cleavage, typical specific gravity 2.8–3.0.

Formed when basaltic lava cooled rapidly at or near Earth’s surface, likely in a volcanic flow; the rounded shape indicates later river, beach, or wave abrasion. Its exact age cannot be determined from the image.

Used as aggregate, road base, riprap, railroad ballast, and dimension stone; attractive polished pieces are occasionally used decoratively but have limited jewelry value.

Basalt is the most abundant volcanic rock on oceanic crust and commonly forms dark, fine-grained lava flows. The holes are vesicles created by gas bubbles escaping during cooling.

Identify by its dense black color, fine texture, lack of visible quartz, and possible weak attraction to a magnet if iron-rich; distinguish from slag by its natural, uniformly rounded form and geological texture. Common worldwide, especially volcanic islands and oceanic regions.