Faint yellow, brilliant pear-cut leucite, 3.03 cts, 12.2 x 8.5 mm, near Ostia, Italy. © The Gem Trader. Used with permission.
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.
| Data | Value |
|---|---|
| Name | Leucite |
| Colors | Colorless, white, gray, yellowish. May appear reddish due to impurities. |
| Crystallography | Tetragonal (pseudo-cubic). Crystals trapezohedral; granular. |
| Refractive Index | 1.50 |
| Luster | Vitreous; dull on some crystals. |
| Hardness | 5.5-6 |
| Fracture | Conchoidal |
| Specific Gravity | 2.47-2.50 |
| Birefringence | Very low to zero. |
| Cleavage | Poor |
| Dispersion | 0.008-0.010 |
| Luminescence | Medium-bright orange in LW (Italy) or none. Blueish glow in X-rays. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long, X-ray Colors |
| Transparency | Transparent to translucent. |
| Phenomena | Some specimens may show color flashes similar to play of color. |
| Formula |
KAISi2O6. |
| Pleochroism |
None. |
| Optics |
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. |
Leucite crystals, Lages, Santa Catarina, Brazil. Photo by Géry Parent. Licensed under CC By-SA 3.0.
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.
If leucite is exposed to the air and other elements, it can pseudomorph (change chemistry without changing shape) into pseudoleucite, a combination of orthoclase feldspar and nepheline. More exposure can transform it further into the clay mineral kaolinite, like these specimens from Brazil. Photo by Eurico Zimbres. Licensed under CC By-SA 2.5.
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 leucites have weak dispersion (0.008-0.010), some specimens may show an unusually high, colorful “fire.” Their internal twinning and striation may cause this peculiar, “play-of-color” type of effect.
A faint yellow, cushion-cut leucite with a flash of color, 1.51 cts, 8.6 x 7.1 mm, near Ostia, Italy. © The Gem Trader. Used with permission.
Scientists have synthesized leucite for geological research. However, there’s no known jewelry use for this synthetic material.
No known gemstone treatments.
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:
Large leucite crystal, 5.0 x 4.4 x 4.3 cm, Magnet Cove, Hot Spring County, Arkansas. © Rob Lavinsky, www.iRocks.com. Used with permission.
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.
Use a soft brush, mild detergent, and warm water for cleaning. Consult our gemstone jewelry care guide for more recommendations.
Leucite, 2.15 cts, Italy. Photo © Joel E. Arem, PhD, FGA. Used with permission.
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