Sodalite

Sodalite forms in silica-poor igneous rocks - specifically nepheline syenites and related alkaline rocks. These unusual magma compositions are deficient in silica but rich in sodium and aluminum, which favors the crystallization of feldspathoids like sodalite instead of the more common feldspars.

The blue color comes from sulfur-bearing color centers within the crystal lattice - a mechanism similar to how lapis lazuli gets its color (lapis contains lazurite, which is chemically related to sodalite). The white veining commonly seen in sodalite specimens is calcite, which crystallized in fractures after the sodalite formed.

A remarkable variety called hackmanite shows tenebrescence - the ability to change color when exposed to UV light. Hackmanite from Myanmar and Afghanistan starts pale or colorless and turns vivid purple-violet under UV exposure, then slowly fades back. This reversible color change is caused by sulfur-related defects in the crystal structure and can be repeated indefinitely.

Sodalite is most often confused with lapis lazuli, as both are opaque and blue. Key differences: sodalite lacks the golden pyrite flecks characteristic of lapis. Sodalite's blue tends toward a slightly more violet tone, while lapis is more purely blue. Sodalite also has a lower specific gravity (2.30 vs 2.70-2.90 for lapis) - it feels noticeably lighter.

At Mohs 5.5, sodalite can be scratched by a steel knife. It may fluoresce orange under UV light (a useful diagnostic). The white calcite veining, while not always present, is a good indicator when it is.