Sphalerite: Gemstone and Jewelry

Sphalerite is a mineral made up of zinc sulfide that is a significant ore of zinc. However, it also happens to make attractive gemstones when it has relatively low levels of iron concentration. Gem-quality sphalerite is available in a range of colors, including red-brown, yellow, green, and pale shades of orange and pink. The colorful nature of sphalerite makes it a favorite for jewelry designers, who are continuously on the lookout for new gemstone varieties to incorporate into their creations. Therefore, in this article, we shall be discussing the gemstone and jewelry of sphalerite, including its properties, gemological features, sources, and market value.

Geological and Mineralogical Overview of Sphalerite: 

Sphalerite, also recognized as zinc blende, is a zinc sulfide mineral that is part of the sulfide mineral group. Its formula is ZnS, which when pure, constitutes 67% zinc and 33% sulfur. Nevertheless, as with any mineral, it primarily forms as a mixture of impurities, trace elements, and structural defects that create a range of colors and properties. It belongs to the isometric crystal class, which means the mineral’s atoms are arranged in a symmetric pattern of 210 different ways.

Sphalerite’s crystal structure comes from a close-packed cubic structure where the zinc atoms take up half of the tetrahedral sites, while the sulfur atoms take up half of the octahedral sites. Sphalerite happens to be one of the most desirable gemstones due to its wide range of colors and birefringence optical property.

Birefringence is an optical phenomenon that happens when light passes through a material and separates into two unique rays that have slightly different angles, leading to a double refraction of the light. According to the Gemological Institute of America (GIA), sphalerite’s birefringence values range from 0.085 to 0.168. This optical nature of sphalerite enables it to exhibit a unique fire effect when viewed under controlled conditions.

Properties and Gemological Features of Sphalerite: 

Sphalerite is a relatively soft mineral, scoring 3.5 to 4 on the Mohs hardness scale. As a result, sphalerite is vulnerable to scratching and chipping and must receive proper handling. Sphalerite’s specific gravity ranges between 3.9 and 4.2 and exhibits either a non-metallic or submetallic luster. Sphalerite’s refractive index ranges from 2.368 to 2.372, with the higher values indicating a greater degree of transparency.

Due to its high birefringence, sphalerite often exhibits a gemological phenomenon referred to as double refraction, whereby light travels through two different paths through the mineral, producing two different refractive indexes, leading to a splitting of the image produced. Colorless sphalerite is relatively rare in nature, and most of the species come in different colors, including yellow, brown, red, orange, green, and black. Color variation in sphalerite results from structural defects within the atomic lattice as well as the presence of trace elements such as iron, manganese, titanium, cadmium, and cobalt.

In addition, sphalerite often undergoes fluorescence when exposed to short-wave ultraviolet light. The colors exhibited by the mineral under such conditions are typically white, pale green, pale yellow, or blue.

Sources of Sphalerite: 

Sphalerite has worldwide distribution and occurs in many countries worldwide. The most significant sources of gem-quality sphalerite include Spain, Mexico, Bulgaria, Slovakia, Russia, and the United States. In Spain, the village of Casapalca, Málaga Province, produces high-quality brown sphalerite for the jewelry market, while the Lavrion district in Greece is known for transparent orange-brown sphalerite crystals.

Mexico, specifically the Naica mine in Chihuahua, produces beautiful specimens of highly transparent pale yellow and reddish-brown sphalerite. In Bulgaria, the Madan ore field produces transparent to translucent gem-quality sphalerite in various color hues such as green, yellow, and orange. Slovakia also produces sphalerite for the gemstone market, with the most sought-after variety featuring a mix of yellow, brown and green colors.

In the USA, gem-quality sphalerite occurs mainly in Franklin, New Jersey. The region has a history of sphalerite mining dating back to the 1800s, and it remains one of the most significant sources of the mineral in the country. Sphalerite gems from this region often exhibit a deep red color coloration due to the presence of iron-impurities. In addition, Missouri in the USA also yields gem-grade sphalerite, usually featuring yellow to orange hues.

Jewelry Uses and Market Value: 

Sphalerite is a popular gemstone in the jewelry industry due to its unique optical properties and wide range of colors. The gemstone is often cut into a variety of shapes, including oval, round, and cushion, and these shapes work to emphasize the stone's inherent beauty. However, sphalerite’s relative softness makes it susceptible to damage in less protective jewelry settings such as rings and bracelets.

Sphalerite’s value as a gemstone depends on several factors such as color, clarity, size, and origin. The gemstone’s rarest and most attractive colors fetch the highest prices. In general, clean and facetable sphalerites with intense and vivid colors command the most significant premiums in the market. The rarer colors include pink, red, and orange hues, which are highly prized by collectors and designers alike. The value of sphalerite is also dependent on its country of origin. For instance, Spanish sphalerites are highly valued due to their intense brown or reddish-brown hues.

Mexican sphalerites are also popular due to their excellent clarity and exceptional yellow color, which often surpasses that of citrine or yellow sapphire. Bulgarian sphalerites, on the other hand, are admired due to their unique color variations, including green and orange. Missouri and New Jersey sourced sphalerites can be undervalued compared to other sources due to the lower intensity of their colors.

Also, the availability of large quality sphalerite gems remains limited, making it a relatively rare gemstone compared to other varieties such as sapphire and garnet. Sphalerite’s rarity contributes significantly to its high value, and top-quality examples can fetch many thousands of dollars per carat. In general, prices for sphalerite range from $50 to $250 per carat for average quality to upwards of $1000 per carat for the most exceptional specimens.

Sphalerite stands out as a beautiful mineral due to its unique range of colors, birefringence optical property, and rarity. Its use as a gemstone dates back several centuries, and it is likely to maintain its relevance in the market for years to come due to its unique combination of properties. However, sphalerite’s relative softness requires due care when handling and wearing it in jewelry designs, and professional maintenance should be consistently practiced to keep it in good condition. Overall, sphalerite is a prized gemstone among collectors and enthusiasts due to its uncommon beauty, making it a valuable addition to the jewelry world.