Although abundant in various lava rocks, leucite is extremely rare in gem-quality form and often has a milky or cloudy look. Faceted stones as well as well-shaped crystals are prized by collectors.
Isotropic: N ~ 1.50. Some specimens may be doubly refractive. Optically (+) if uniaxial. May be anomalously biaxial.
Optic Sign
Uniaxial +
Etymology
From the Greek leukos, meaning "white."
Occurrence
In potassium-rich basic lavas.
Inclusions
Crystals of apatite, augite, magnesite, natural glass, olivine, and spinel.
Does Leucite Make a Good Jewelry Stone?
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 actually makes them resistant to splitting. Nevertheless, protective settings are recommended, especially for ring use.
Because of their sometimes white, milky appearance and outwardly isometric shapes, leucites have been called “white garnets.” However, leucite and garnet are distinct gem species with very different refractive indices (RI), specific gravity (SG), and hardness.
Although colorless grossular garnet is called “leuco garnet,” that only means this stone shares an etymology with leucite. Leukos is the Greek word for “white.”
Identifying Characteristics
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.
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.
Some gems are seldom faceted because they’re rarely found in nature or rarely used for jewelry. Learn a few tricks for faceting seven exotic gemstones.…
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