Sodalite

Hardness: 5.5-6 on Mohs scale. Color: Typically deep blue, but can also be grayish, greenish, or yellowish. Often mottled with white patches (calcite). Luster: Vitreous to greasy. Crystal structure: Isometric system, usually massive, granular, or in disseminated grains; rarely forms dodecahedral crystals. Cleavage: Poor in six directions. Specific Gravity: 2.13-2.34.

Sodalite forms in silica-poor igneous rocks, such as nepheline syenites, phonolites, and trachytes. It can also be found in metamorphic rocks that have undergone metasomatism. The formation involves crystallization from magmas that are undersaturated in silica, often at relatively high temperatures and pressures. Geological age varies depending on the host rock, ranging from Precambrian to Cenozoic.

Sodalite is primarily used as an ornamental stone, particularly in jewelry (beads, cabochons) and carvings. It is also used in decorative objects, inlays, and as a facing stone. Its attractive blue color makes it a popular choice for lapidary work.

Sodalite is a member of the sodalite group of minerals. It can be confused with lapis lazuli, but sodalite typically lacks the pyrite inclusions common in lapis lazuli and is a simpler blue. Some sodalite exhibits tenebrescence (also known as photochromism or reversible photochromism), meaning it can change color when exposed to light, deepening in color or shifting from white to pink/red when first extracted and then fading upon exposure to strong light.

Sodalite is commonly found in Brazil (Bahia), Canada (Ontario), Greenland, Russia (Kola Peninsula), and the USA (Maine, Arkansas). In the field, it can be identified by its distinctive blue color, often with white streaks or patches of calcite, and its moderate hardness. A key identification test is its reaction to hydrochloric acid: sodalite will produce a faint smell of hydrogen sulfide when powdered and exposed to HCl, due to the sulfur impurities within its crystal structure (though this is often subtle). Its massive habit and greasy luster are also good indicators.