Andradite is the calcium iron member of the garnet group. Its best-known gem variety, demantoid, is a green garnet with dispersion higher than diamond, so it throws strong flashes of spectral color. Andradite also includes black melanite and yellow to brown topazolite. Demantoid from Russia's Ural Mountains, with its fine "horsetail" inclusions, is among the most prized garnets.
What andradite is
Andradite has the ideal formula Ca3Fe2(SiO4)3, with calcium in the X site and ferric iron in the Y site. It belongs to the ugrandite series of garnets, alongside uvarovite and grossular, and it forms solid solutions with grossular. The species is named after the Portuguese mineralogist Jose Bonifacio de Andrada e Silva, who described one of its varieties around 1800.
Andradite is cubic and singly refractive. It has the highest refractive index of the common garnets, about 1.88 to 1.89, which is above the range of a standard refractometer. Its dispersion, about 0.057, is higher than diamond's 0.044. That is why a well-cut demantoid can show more fire than a diamond of the same size, although its green body color masks some of it. Hardness is about 6.5 to 7, softer than most garnets. The diamond optical properties page explains dispersion and how it compares.
Demantoid
Demantoid is transparent green andradite colored by chromium, sometimes with iron contributing yellow. Its name comes from an old Dutch or German word for diamond, a nod to its fire and luster. Colors range from yellowish green to an intense emerald green. Fine demantoid shows a pure, saturated green with visible fire; yellower and paler stones show more dispersion but less color.
Horsetail inclusions
Many demantoids, especially from Russia, contain bundles of fine, curved fibers radiating from a small crystal. These "horsetail" inclusions are fibers of chrysotile, a serpentine mineral. They are so typical of Russian demantoid that collectors seek them out, and they can add value instead of lowering it. Horsetails are rare in other gems, so they also help with identification. They form because Russian demantoid grew in serpentinite, a rock rich in serpentine minerals.
Russian Urals
Demantoid was discovered in the Ural Mountains of Russia in the mid nineteenth century, in alluvial deposits along the Bobrovka River, which became its best-known source. It became popular in Russian and European jewelry in the late nineteenth and early twentieth centuries. Peter Carl Faberge's workshops and other Russian jewelers used it, and demantoid set in small clusters, brooches and insect motifs was common in Edwardian pieces. Production was interrupted by the Russian Revolution and later revived on a smaller scale.
Namibia, Madagascar and other sources
In the 1990s, demantoid was found in Namibia, in skarn deposits rather than serpentinite. Namibian stones are often clean, with fewer horsetails, and include fine green material. Madagascar became a source in the late 2000s, producing yellowish to green demantoid with some horsetail-type inclusions. Demantoid has also come from Italy's Val Malenco, Iran, Pakistan and Afghanistan.
Treatment
Some demantoid, especially from Russia and Madagascar, is heated at low temperatures to reduce brownish or yellowish tones and improve the green. The treatment is usually undetectable and stable. See gemstone treatments explained for more on heating.
Melanite
Melanite is black andradite, colored by titanium and iron. It forms in igneous rocks rich in alkali, such as some volcanic rocks in Italy and Germany. Melanite takes a bright, almost metallic polish and has occasionally been cut as a black gem, though it is mainly a collector's mineral. Black stones in Victorian jewelry were more often jet, onyx or glass. Titanium-rich andradite is sometimes called schorlomite when titanium content is high enough to define a separate species.
Topazolite
Topazolite is yellow to yellowish brown andradite, named for its resemblance to topaz. It usually forms small, sharp crystals in cavities, and facetable pieces are rare and tiny. Collectors value it, but it rarely appears in commercial jewelry.
How andradite forms
Andradite forms mostly in skarns, metamorphic rocks created where hot fluids from an intrusion react with limestone or dolomite. It also grows in serpentinites, as with Russian demantoid, and in some alkaline igneous rocks, as with melanite. The metamorphic rocks and gemstones page explains skarns and other settings.
Identification
Demantoid can resemble tsavorite, green tourmaline, chrome diopside, peridot and emerald. Its high refractive index, single refraction, strong dispersion and horsetail inclusions separate it. Under a Chelsea filter, chromium-rich demantoid can appear red. A hand spectroscope shows a distinctive iron band in the blue-violet.
Value
Fine demantoid is one of the most expensive garnets. Value rises with saturated green color, visible fire and size. Most demantoid is small; clean stones over 2 carats with fine color are scarce. Russian origin and attractive horsetails can add a premium among collectors. The how colored gemstones are graded page explains how color, clarity and origin affect value.
Wear and care
Andradite is softer than other garnets at about 6.5 to 7, so demantoid can show abrasion on facet edges with rough wear. It has no cleavage. In rings, protective settings help. Earrings and pendants need little special care. Clean demantoid with warm soapy water and a soft brush. Avoid ultrasonic cleaning for stones with many inclusions, and avoid steam and heat. The jewelry cleaning guide has more advice.
Frequently asked questions
Is demantoid more fiery than diamond?
Demantoid's dispersion, about 0.057, is higher than diamond's 0.044. Its green color masks some of the fire, but fine stones show strong flashes.
What are horsetail inclusions?
They are curved fibers of chrysotile radiating from a central point. They are typical of Russian demantoid and are prized by collectors.
Is demantoid treated?
Some demantoid is gently heated to improve green color. The treatment is stable and often undetectable.
What is melanite used for?
Melanite is black andradite. It is occasionally cut as a black gem but is mostly a collector's mineral.