Leucite crystallizes into an isometric structure at very high temperatures (~900° C). However, as it cools, it recrystallizes into a tetragonal structure while retaining an outwardly isometric shape. Since tetragonal crystals have axes of two different lengths, some leucites have a very weak birefringence. (In contrast, isometric gems are singly refractive). This transformation into a tetragonal structure also frequently causes striation and twinning.
Because of their sometimes white, milky appearance and outwardly isometric shapes, leucites have also been called “white garnets.” However, leucite and garnet are distinct gem species with very different refractive indices (RI), specific gravity (SG), and hardness. Also note, while the colorless grossular garnet variety is called “leuco garnet,” that only means it shares an etymology with leucite.
Any leucites over 3 carats in size will likely contain inclusions.
Although this mineral occurs in many localities all over the world, to date, only Italy has produced transparent, facetable leucite crystals. The Alban Hills near Rome produce transparent, colorless crystals up to about 1 cm in size.
Mount Vesuvius (Monte Somma) is the type locality of this mineral.
Notable sources of fine crystals include the following:
United States: Arkansas; Montana; New Jersey; Wyoming.
Australia; Brazil; British Columbia, Canada; Democratic Republic of the Congo; France; Germany; Tanzania; Uganda.
Faceters have cut Italian material to about 3 carats. This facetable material is scarce, always small, and clean only in very tiny stones.
You’re more likely to find faceted leucites in mineral collections than jewelry collections. Although some stones may have unusual color flashes, their general appearance doesn’t typically lend itself to jewelry wear. They also only have a hardness of 5.5 to 6, which makes them susceptible to scratching from household dust. On the other hand, they have poor cleavage, which makes them resistant to splitting. Nevertheless, protective settings are recommended, especially for ring use.