Sphalerite is a zinc sulphide mineral that is quite rare in gem quality. Top grade specimens are prized for their exceptional fire or dispersion, which is higher than that of diamond.
Since sphalerite is a relatively soft stone, with a hardness of only 3.5 to 4 on the Mohs scale, it is not suitable for rings. It can be used in pendants if set carefully. But it is mainly a gem for the rare stone collector.
Sphalerite is the chief ore of zinc, and gem quality specimens are sometimes found in zinc mines. But most sphalerite contains iron, and it is black in color and completely opaque when the iron content is high. The rare gem quality specimens have a low iron content and a high degree of transparency. Colors range from yellowish to golden brown, orange, red and green.
Gemologically, sphalerite forms as cubic crystals, like diamond, and has perfect cleavage in six directions. It has a refractive index of 2.368 to 2.371, just slightly less than diamond, and higher than demantoid garnet, sphene, zircon and corundum. It is one of the few gems with an admantine or diamond-like luster. But it is sphalerite's dispersion or fire that is truly unique -- it has a dispersion rating of 0.156, significantly higher than demantoid garnet (0.057), sphene (0.051) and diamond (0.044). Sphalerite has a density or specific gravity of 3.9 to 4.1, about the same as sapphire.
Sphalerite is usually given a brilliant cut or Portuguese cut to maximize its brilliance and fire. Sphalerite is usually completely untreated.
Some of the finest and largest sphalerite in the world comes from the Aliva mine in the Picos de Europa Mountains in the Cantabria region on the north coast of Spain. Zinc was mined in this region for many years, and when zinc mining was no longer economic the mine produced some exceptional transparent toffee-colored sphalerite in big sizes. These extraordinary specimens became well-known by collectors of rare gems, and fine faceted Spanish sphalerites can be found in major museums and private gem collections.
The Aliva mine was closed in 1989 and the mine is now part of a national park. That is why the Spanish sphalerite is now quite scarce -- all of the gems in the market were cut from material mined years ago. Finding large clean stones of good color is now very difficult. Though sphalerite is found in other locations around the world, such as Mexico, Peru and Bulgaria, the Spanish sphalerite still has no rivals for quality.
See our collection of Rare Sphalerite from Spain
Notable Gems from the AJS Collection
This month we feature new acquisitions to our collection. Click on a photo to see the details for the item.
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6.32 ct Spinel from Mahenge, Tanzania [SOLD]
A gorgeous pink-red Mahenge spinel in a rare large size! This extraordinary gem has excellent clarity, fine cutting and wonderful briliance. Certified by GemResearch Swisslab (GRS) as natural and untreated, with the color graded as "pinkish-red". A superb gem with the neon glow characteristic of the rare spinel from Mahenge, Tanzania.
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4.18 ct Blue Sapphire from Sri Lanka
A fully saturated fine Ceylon royal blue sapphire. This stone was precision cut with excellent polish. Certified by GemResearch Swisslab (GRS) with the color graded as "vivid blue (GRS type "royal blue")." A special gem that will surely make a beautiful piece of jewelry for someone special. |
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An outstanding large sphalerite from the Aliva mine in northern Spain. This gorgeous round sphalerite is over 14 mm in diameter and is amazingly bright and fiery, with flashes of orange, yellow and green as it is turned in the light. Beautifully cut and very clean, this is a superb gem sphalerite for your collection. Guaranteed natural and untreated. |
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An impressively large gem scapolite from Tanzania! Completely clean, this fine material has been precision cut to produce a dazzling gem with superb luster. This will set up beautifully in a cocktail ring or pendant, or as a fine addition to your gem collection. Guaranteed natural and untreated. |
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2.05 ct Ruby from Burma
A very fine Burmese ruby with outstanding color and clarity. Fine Burmese rubies over 2 carats are now extremely rare. This gem is a deep saturated red and is beautifully cut to maximize brilliance. The clarity of this material is simply wonderful for ruby, and we have graded this gem "almost loupe clean". Certified as "pigeon's blood red" by GRS. This is a gem that you will enjoy for years and your family will treasure for generations. |
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17.10 ct Barite from Africa [SOLD]
Barite (or Baryte) is a barium sulfate mineral that is not commonly found cut as gemstones. It has a hardness of 3-3.5 on the Mohs scale. This impressively large Barite has been precision cut and will make a fine addition to a gem collection. Guaranteed natural and untreated.
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News from AJS and the Gems World
- Hope Spinel sold at Auction for First Time in 98 Years
Henry David Hope was a banker and gem collector who died in 1839. He is best known for the Hope Diamond, a 45.52 carat deep blue diamond (now in the collection of the Natural History Museum in Washington, D.C.). Among the important stones in his 700 piece collection was a 50.13 ct spinel, known as the Hope Spinel. This rare large spinel last appeared at auction at Christie's in 1917. The gem sold at Bonhams London auction on Sept. 24 for $1.5 million, setting a new auction record price per carat for spinel.
- Sotheby's to Auction 12 carat "Blue Moon" Diamond
In January 2014 Petra Diamonds announced the discovery of a 29.62 carat blue rough diamond at their Cullinan mine in South Africa. They have now cut a 12.03 carat cushion diamond from the stone, which they are calling the "Blue Moon" Diamond. GIA has graded the stone as "Fancy Vivid Blue", the highest color grade for blue diamonds, with a clarity of IF. Sotheby's will offer the gem at auction in Geneva on November 11, with a pre-sale estimate of $35 to $55 million.
Ask the Gem Experts
Each month we answer questions from our customers. We welcome your questions and you can submit a question from our contact page.
What are the rarest gemstone varieties in the world? GR, USA
All gemstones are rare, since they constitute a tiny percentage of the minerals found on earth. But some gemstones varieties are extremely rare, insofar as only a small number of gem-quality specimens are known to exist. They include rare species such as painite, taaffeite, grandidierite, jeremejevite, poudretteite and benitoite, as well as very rare colors of more common gems such as red diamond and red beryl. For more information, see our article on the Rarest Gemstones in the World.
What is the most abundant mineral in the world? I imagine it must be something like quartz? SM, UK
Quartz is indeed one of the most common minerals, but it is not the most abundant. The feldspars are the most common mineral in the earth's crust, comprising about 60% of all igneous rocks. Plagioclase feldspar gemstones include moonstone, sunstone, oligoclase, orthoclase, labradorite, andesine and amazonite.
If we consider the earth's mantle -- much thicker than the earth's crust -- the most common mineral is olivine, a magnesium iron silicate. Gem quality specimens are known as peridot. There are several olivine minerals, including wadsleyite, ringwoodite and bridgmanite. Bridgmanite, which is found in the lower mantle at depths between 670 and 2,900 kms is, according to geologists, the most abundant mineral in the world.
All the best in gems,
Arnold, Rung & Ron
