Sodalite

Hardness: 5.5-6 on Mohs scale. Color: Typically deep royal blue, often with white streaks or patches (due to calcite). Can also be gray, yellow, green, or pink. Luster: Vitreous to greasy. Crystal structure: Isometric (dodecahedral crystals, but often massive or granular). Cleavage: Imperfect to distinct in six directions. Specific Gravity: 2.13-2.29.

Sodalite is a tectosilicate mineral found in silica-poor igneous rocks such as nepheline syenites, phonolites, and trachytes, and also in some skarns and hydrothermal veins. It forms during the late stages of magmatic crystallization or through metasomatic processes. Its geological age varies depending on the specific igneous intrusions where it occurs, but it is often found in geological formations dating back hundreds of millions of years.

Sodalite is primarily used as an ornamental stone and in jewelry, often cut into cabochons, beads (as seen in the image), and carvings. Due to its attractive blue color and relatively good hardness, it's a popular alternative to lapis lazuli. It is also used for decorative purposes in architecture (e.g., countertops, inlays) and as a collector's mineral specimen. Historically, deep blue sodalite was sometimes used as an architectural facing stone.

Sodalite is one of the components of Lapis Lazuli, although lapis lazuli is a rock rather than a single mineral. It is known for its ability to fluoresce orange under ultraviolet light, which can be a key identification feature. The deep blue variety is often called 'royal blue sodalite'. Hackmanite, a rare variety of sodalite, exhibits tenebrescence (or reversible photochromism), meaning it changes color when exposed to sunlight and then reverts to its original color in the dark.

Sodalite is commonly found in Brazil (Bahia), Bolivia, Canada (Ontario, Quebec), Greenland, India, Italy (Vesuvius), Namibia, Russia (Kola Peninsula), and the USA (Arkansas, Maine, Montana, New Hampshire). In the field, look for its distinctive blue color, often intermixed with white calcite (which can effervesce with dilute acid). It typically occurs in massive forms or as disseminated grains within igneous rock. Its relatively low hardness (compared to quartz) and vitreous to greasy luster can help distinguish it. Testing for fluorescence under UV light can be very helpful for identification if equipment is available.