How to Identify Basalt: What to Look For and Home Tests

Lava that cooled at the surface had no time to grow large crystals, so the rock is fine-grained or glassy, often with gas bubbles or a frothy top surface. The properties that identify Basalt — color: Dark-colored, gray to black. For a rock, texture and grain size matter more than any single hardness number.

• Composition test: contains 45–53% silica and is rich in iron and magnesium.
• Surface-texture test: basaltic lava flows commonly show smooth, ropy/billowy surfaces (pāhoehoe) or rough, sharp, clinkery surfaces (ʻaʻā). Safety: Solidified ʻaʻā lava has surfaces as sharp as knife blades. Wear thick-soled shoes and gloves when handling it in the field.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

Basalt is a rock and has no single Mohs hardness or specific gravity value. Its low silica content gives basaltic lava low viscosity and high fluidity (source: NPS, 'The low silica concentrations in these lavas mean that they are highly fluid').