An unusually large, faceted phosgenite with a rich cognac-like hue from the Monteponi Mine, Sardinia, Italy. 37.66 carats, modified rectangle brilliant cut. © Rob Lavinsky, www.iRocks.com. Used with permission.
Rare phosgenite typically shows pale colors. This material is difficult to cut and too soft for jewelry wear. However, its strong yellowish fluorescence appeals to collectors of unusual gemstones.
| Data | Value |
|---|---|
| Name | Phosgenite |
| Crystallography | Tetragonal; Crystals prismatic, thick and tabular; massive, granular. |
| Refractive Index | 2.114-2.145 |
| Colors | Colorless, white, yellowish white, gray, shades of brown, greenish, pinkish. |
| Luster | Adamantine to greasy. |
| Polish Luster | Adamantine. |
| Hardness | 2-3, varies with direction. |
| Fracture | Conchodial |
| Specific Gravity | 6.13. |
| Birefringence | 0.028 |
| Cleavage | Distinct 1 direction, Good |
| Luminescence | Strong yellowish fluorescence in UV and X-rays. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, X-ray Colors |
| Transparency | Transparent to translucent. |
| Formula | Pb2CO3Cl2. |
| Pleochroism | Very weak, reddish/greenish, only in thick pieces. |
| Optics | o = 2.114-2.118; e = 2.140-2.145. Uniaxial (+). May be anomalously biaxial. |
| Optic Sign | Uniaxial + |
| Etymology | From phosgene, a name for the compound COCl2 (carbonyl chloride) because the mineral contains C, O, and Cl. |
| Occurrence | A secondary mineral in lead ore deposits. |
“Phosgenite,” Wallclose Mine, Cromford, Derbyshire, England, UK, from the collection of the British Museum of Natural History. Photo by Rock Currier. Licensed under CC By 3.0.
Lapidaries can cut massive phosgenite into interesting cabochons of various colors, up to the size of the rough (several inches). However, faceted pieces are more rarely encountered, not simply due to the material’s scarcity. Phosgenites can have directions of variable (and low) hardness (2-3) and distinct to good cleavage. Although this gem can have an adamantine, diamond-like luster, it’s also difficult to polish. If you take into account its flexible and sectile properties, it just doesn’t make a practical jewelry stone.
While phosgenite usually occurs colorless or in soft shades of yellow, gray, brown, pink, or green, it has a strong yellowish fluorescence under both ultraviolet light (UV) and X-rays. This fluorescence and its relatively high specific gravity (SG) of 6.13 help to distinguish it from stones of similar range and hardness. In fact, phosgenite has a much greater density than two more popular gems with a similar range of colors and hardness: amber (1.08) and mellite (1.64).
However, two other rarely faceted collector’s gemstones have a comparable range of colors, hardness, and SG. Like phosgenite, anglesite and cerussite can be colorless as well as white, grayish, yellowish, or greenish. Their fluorescence under UV light can also appear yellowish. Furthermore, these minerals can crystalize in close association.
“Phosgenite with cerussite,” Thorikos Bay slag locality, Thorikos area, Laurium District slag localities, Laurium District, Attica Prefecture, Greece. Photo by Didier Descouens. Licensed under CC By-SA 4.0. (The transparent prismatic crystals are phosgenites; the acicular, “needle-like” crystals are cerussites;).
| Hardness | SG | Fluorescence in UV | Refractive Index | Optic Character | |
| Anglesite | 2.5-3 | 6.30-6.39 | Weak yellowish. | a = 1.877; b = 1.883; γ = 1.894 | Biaxial (+) |
| Cerussite | 3-3.5 | 6.55 | Can be yellow in LW. Pale blue/green in SW. | a = 1.804; b = 2.076; γ = 2.079 | Biaxial (-) |
| Phosgenite | 2-3 | 6.13 | Strong yellowish. | o = 2.114-2.118; e = 2.140-2.145 | Uniaxial (+) |
A refractive index and an optic character reading with a refractometer and/or polariscope may be the most effective way to distinguish these gems. However, keep in mind that phosgenites may be anomalously biaxial. Thick pieces of phosgenite (not all specimens) may have weak pleochroism, while anglesite and cerussite don’t.
The yellow fluorescence of this gemmy phosgenite crystal on galena is more marked in its upper centimeter. 4.2 x 3.0 x 3.0 cm. Monteponi Mine, Iglesias, Sardinia, Italy. © Rob Lavinsky, www.iRocks.com. Used with permission.
Scientists have synthesized phosgenite for various purposes for centuries. In the February 2014 newsletter of the British Micromount Society, Steve Plant describes various manufacturing methods, from the time of the Ancient Egyptians (who used synthetic phosgenite for makeup) through 19th and early 20th century procedures to current techniques. Although lab-created crystals do exist, there is no known jewelry use for this material.
None known.
Monteponi, Sardinia, Italy produces fine, yellow-brown crystals up to 5 inches across, some with facetable areas.
Other notable sources include the following:
2.89-ct phosgenite, dark brown “cocoa,” emerald radiant cut, 7.5 x 5.8 mm, Morocco. © The Gem Trader. Used with permission.
Rare faceted specimens almost always weigh less than 2 carats. Typically from Sardinia, these gems usually have a yellowish brown color. However, a few larger stones exist, some up to about 10 carats. (The 37.66-ct faceted phosgenite at the top of the article is a giant for this species).
Phosgenites contain lead. When cutting this material, avoid ingesting or inhaling particles and wash your hands.
Jewelry use is not recommended.
Phosgenite: Monteponi, Sardinia, Italy (1.5). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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