General Gemstone Details
Black, brown, or green on thin, brittle, translucent edges, a glassy luster with uneven cleavage. Visually recognizable only when crystallized. Usually in prismatic, embedded crystals, several inches (10 cm) long, terminated by steep points, as well as in fibrous masses.
Aegirine is common in high-soda, low-silica rocks, in the fine-grained equivalents. This entire family of high-soda, low-silica rocks, however, is quite uncommon. The best U.S. occurrence is at Magnet Cove, Arkansas, where slender crystals several inches (10 cm or more) long are very abundant. In Canada, St. Hilaire in Quebec has become famous for its feldspathoid group of hundreds of nepheline syenite associates.
In Russia, the Kola Peninsula is equally famed for a like assemblage, commonly with rare-earth, zirconium, and titanium minerals. Acmite needles are set in a dark-hued dike of this material at Beemerville, New Jersey. Mid-size crystals are found in low-silica rocks in the Bear Paw and Highwood mountains in Montana and near Colorado Springs. The same sort of rock is found in Greenland; at Langesundfjord, Norway; and near Poços de Caldas, Brazil.