Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks that crystallized from sodium-rich magmas. This is the origin of the name "sodalite." The name sodalite comes from the Greek words soda and lithos, which means salt stone, as the stone contains a lot of salt.
Sodalite is a rich royal blue tectosilicate mineral widely used as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. High-quality sodalite is used as a gemstone, a sculptural material, and an architectural stone.
Although somewhat similar to lazurite and lapis lazuli, sodalite rarely contains pyrite (a common inclusion in lapis) and its blue color is more like traditional royal blue rather than ultramarine. It is further distinguished from similar minerals by its white (rather than blue) streak.
Sodalite can be strongly fluorescent, often fluorescing bright orange to red, or cream. It may also be tenebrescent, in that its color will deepen upon exposure to ultraviolet light. This is especially true of the purple variety Hackmanite which is found principally in Mont-Saint-Hilaire and Greenland.
Sodalite can be identified by its deep blue color and strong orange fluorescence. Sodalite tends to exhibit a vitreous luster with a greasy luster on fractures. It can sometimes exhibit a violet tint and in opaque form often has white veins or patches running through it, which are composed of calcite. Sodalite is rare in transparent crystal form and can be gray, yellow, green, orange, blue, violet, pink, or colorless.
Found in the Kangerlussuaq area, green sodalite is a vibrant green under daylight, when exposed to SW UV is very tenebrescent. The Tareq Slopes offer many varieties of sodalite but none match the beauty of pieces which are a gemmy yellow color in daylight. In addition to their brilliance under UV, these pieces exhibit a deep purple tenebrescent - one of the deepest color changes of all the minerals from the complex. The southern shore of the Tunuliarfik Fjord yields unusual blue sodalite which is quite tenebrescent and very fluorescent. Red sodalite is found around the same area as the blue sodalite which appears red in daylight. The red color is due to the extreme tenebrescence of this material. When exposed to sunlight it turns a reddish color. Some sodalite has the outward appearance of albite but can glow a multitude of colors. Greenland sodalite is found throughout the various complexes in large quantities. Much of it is the typical gray or yellow variety, and almost all of it fluoresces a bright orange.
First discovered by Europeans in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.
Significant deposits of fine material are restricted to but a few locales: Bancroft, Ontario, and Mont-Saint-Hilaire, Quebec, in Canada, and Arkansas, in the US. Smaller deposits are found in South America (Brazil and Bolivia), Portugal, Romania, Burma, and Russia.
Sodalite is not a birthstone though sometimes it is used as the birthstone of December instead of turquoise.