Tumbled sodalite with white calcite veining

The specimen appears to be a small polished or naturally rounded piece with deep navy-blue to blue-gray color and contrasting white-gray banding or veining. Sodalite has a vitreous to greasy luster, Mohs hardness of about 5.5–6, a cubic crystal structure, uneven to subconchoidal fracture, and generally poor cleavage. Specific gravity is approximately 2.27–2.33. The visible white material is most likely calcite, though other feldspathoid or alteration minerals cannot be ruled out from the photograph alone.

Sodalite forms mainly in silica-poor, alkaline igneous rocks such as nepheline syenite and phonolite, and sometimes in associated metamorphic or metasomatic zones. The blue color results from sulfur-related defects in the sodalite structure. The specimen’s exact geological age cannot be determined visually; sodalite deposits range from Precambrian to much younger geological formations.

Used as an ornamental and lapidary stone for tumbled stones, cabochons, carvings, beads, and decorative objects. It is popular with mineral collectors and is sometimes used as a less-expensive alternative to lapis lazuli. It has little major industrial use because of its moderate hardness and limited abundance.

Sodalite is commonly confused with lapis lazuli because both are blue and may contain white calcite. Lapis lazuli, however, is a rock made of several minerals and often contains golden pyrite, while sodalite is a distinct mineral that commonly has white calcite veins. Polishing can make the blue color appear darker and more uniform than it is in the rough.

Likely identification clues are the intense royal-to-navy blue color, white calcite veining, opaque appearance, and relatively soft surface compared with quartz. A steel knife may barely scratch sodalite, while quartz should not scratch it easily; calcite veins can be scratched much more readily. A streak test on an inconspicuous area may produce a pale blue or white streak. Confirmation would require hardness, specific-gravity, refractive-index, or analytical testing. Common sources include Ontario, Canada; Brazil; Greenland; Namibia; Russia; and the United States, especially Arkansas and Montana.