Basalt (River Rock/Cobble)

Hardness: 6-7 on Mohs scale (for quartz), Color: Typically gray to dark gray, but can vary depending on mineral content. Luster: Dull to vitreous for constituent minerals. Crystal Structure: Fine-grained to aphanitic texture, individual crystals usually not visible to the naked eye. No distinct cleavage due to fine grain size.

Formed from the rapid cooling of basaltic lava erupted at the Earth's surface or underwater. It is a volcanic extrusive igneous rock. Its age varies greatly depending on the volcanic activity that formed it, ranging from recent to billions of years old.

Commonly used as crushed stone for construction aggregate (roads, concrete), building stone, and in some cultures, for traditional tools or ornaments. River-worn basalt (like the one pictured) is often used in landscaping, as decorative stones, or as 'wishing stones' / 'worry stones'.

Basalt is the most common rock type in the Earth's crust, making up oceanic crust. It is also found on other terrestrial planets like Mars and the Moon. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt.

Look for a dark, fine-grained, heavy rock, often with visible small vesicles (gas bubbles) or a somewhat rough texture. When found in rivers or beaches, it will be smooth and rounded due to abrasion. In the field, it's identified by its dark color, density, and volcanic origin clues (e.g., presence of lava flows). Common in active and ancient volcanic regions worldwide, especially along mid-ocean ridges, volcanic islands (e.g., Hawaii, Iceland), and continental flood basalt provinces.