Basalt with Calcareous Encrustations
Basalt with Calcium Carbonate (Calcite) surface deposits · Mineral Specimen / Common Igneous Rock
Crystal System
Amorphous to Microcrystalline (Basalt); Trigonal (Calcite encrustations)
Mineral Group
Silicates (Mafic Volcanic Rock) containing Carbonate surface minerals
Chemical Formula
Mainly (Na,Ca)Al1-2Si3-2O8 (Plagioclase) + (Mg,Fe,Ca)2Si2O6 (Pyroxene) + CaCO3 (Encrustation)
Formation Process
Igneous (Extrusive); formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface
About This Crystal
A fine-grained, dark grey to black volcanic rock with a dull, earthy luster. The surface is heavily encrusted with white to tan, irregular patches of calcium carbonate/marine organisms (serpulids or bryozoans) indicating a period spent underwater or in a lime-rich environment.
Physical Characteristics
Color: Charcoal black; Streak: Dark grey; Cleavage: None (subconchoidal fracture); Specific Gravity: 2.8–3.0; Magnetism: Often weakly magnetic due to magnetite content; Fluorescence: Generally none.
Optical Properties
Opaque; No visible pleochroism or dispersion; Luster is typically dull or sub-metallic in fresh breaks, but weathered here.
Hardness & Durability
Origin Region
Global; common in oceanic crust, volcanic islands (Hawaii, Iceland), and tectonic plate boundaries
Hardness & Durability
Mohs 5-6 (Basalt matrix); Encrustations are softer (Mohs 3). Very tough and durable as a rock, though encrustations are brittle and prone to flaking.
Care & Maintenance
Avoid acidic cleaners (which dissolve the white carbonate surface); clean with warm soapy water and a soft brush. Durable and light-stable.
Rarity & Value
Very Common; primarily of geological or decorative interest rather than gemstone value. $5 - $20 for typical landscape or teaching specimens.
Special Characteristics
Vesicular texture (small holes/pores) from gas bubbles during formation; biological encrustations suggest a marine or aquatic history.
Lore & History
Historically used as a foundational building material and for grinding stones. Metaphysically associated with grounding, strength, and stability due to its volcanic origin from the Earth's core.