Andradite

Garnet · also called Demantoid, Topazolite, Melanite, Allochroite, Colophonite

Andradite is the calcium-iron garnet, Ca₃Fe₂(SiO₄)₃, a cubic mineral whose dispersion of 0.057 is higher than diamond's. Its varieties span the price range of the garnet group, from chromium-green demantoid at the top through yellow topazolite to opaque black melanite. Refractive index reads beyond a standard refractometer.

What is andradite?

Andradite is the calcium-iron member of the garnet group, Ca₃Fe₂(SiO₄)₃, a cubic mineral that includes some of the most valuable garnets cut as well as some of the least. It is the garnet species with the highest dispersion, higher than that of diamond.

The species is not a single gem but a family of them. Its best-known variety is demantoid, the chromium-bearing green material from the Urals and later from Namibia, Iran and Madagascar. Alongside it sit topazolite, a yellow to yellow-green variety, and melanite, an opaque black titanium-bearing form. Andradite belongs to the ugrandite branch of the garnet group with grossular and uvarovite, and it forms a chemical series with grossular, so intermediate compositions are common and analytical figures shift along it.

What gives andradite its colour?

Ferric iron is an essential structural component rather than an impurity, and it supplies the yellow-green to brownish base colour of the species. The strong greens of demantoid come from small amounts of chromium, with vanadium contributing in some deposits, while titanium produces the black of melanite.

The brownish component that dulls much andradite is also iron-related, and it is what separates a fine demantoid from an ordinary one. Where chromium is present in quantity and iron-related brown is subdued, the result is the vivid grass green the variety is known for; where chromium is scarce, the same mineral is a yellow or brownish yellow topazolite. Because the composition varies continuously towards grossular, so does the colour, and a single pocket can produce material across the range.

How is andradite identified?

Refractive index is 1.880 to 1.888, which is beyond the range of a standard refractometer reading to about 1.81, so the practical result is an over-the-limit reading rather than a number. Specific gravity is 3.80 to 3.90, the stone is singly refractive, and dispersion is 0.057.

That combination is unusual enough to be conclusive. An over-the-limit reading with a specific gravity near 3.84 and visible fire eliminates most of the field at once. Among garnets, grossular reads on the scale at about 1.73 to 1.75, pyrope near 1.74 with a lower specific gravity, and spessartine sits near the limit with a specific gravity above 4.1. Under crossed polarising filters andradite is isotropic but very commonly shows anomalous double refraction as strain patterns, which is normal for the species and not evidence against it. The stones most often confused with fine green andradite are chrome tourmaline, tsavorite and peridot, all of which are doubly refractive and read well below the refractometer limit.

The distinguishing feature: dispersion above diamond's

Andradite disperses white light more strongly than diamond does, 0.057 against 0.044, and in a well-cut pale or medium-toned stone that shows as unmistakable flashes of spectral colour between the facets. It is the single property that identifies the species across a room.

The fire is easiest to see in lighter material. In a deeply saturated green demantoid, body colour absorbs much of the dispersed light and the fire is subdued, which is why some of the most obviously fiery andradites are the yellow topazolites and the paler greens rather than the most expensive stones. Cutters take advantage of this by using brilliant-style crowns and shallow enough pavilions to keep light moving. A stone advertised as demantoid that shows no fire at all under a point light source deserves a second look, since the property is intrinsic to the species and does not depend on origin.

What are the varieties of andradite?

Four names cover most of what is traded. Demantoid is the chromium-bearing green variety and by a wide margin the most valuable. Topazolite is yellow to yellow-green and transparent. Melanite is black and titanium-rich. Rainbow andradite is an iridescent material with no separate chemical identity.

Demantoid is treated at length in its own entry. Topazolite occurs in small clean crystals, historically from the Piedmontese and Lombard valleys of the Italian Alps, and is cut mainly for collectors because the rough is rarely large enough for jewellery. Melanite is opaque, was used in nineteenth-century mourning jewellery, and occurs in alkaline volcanic rocks. Rainbow andradite, from Japan and Mexico, shows iridescent films caused by fine lamellar structures within the crystal that interfere with light; it is usually cut as cabochons or left as specimens, and the effect, not the body colour, is the point.

Is andradite treated?

Demantoid is routinely heated at modest temperatures to reduce a brownish component and improve the green, a treatment documented by GIA and one that is not always disclosed. It leaves no reliable visible trace, which is the reason to buy significant stones with a laboratory report rather than on assurance.

Beyond that, treatment is uncommon. There is no established diffusion, irradiation or dyeing practice for the species, and the fracture-filling that affects some other coloured stones has not become routine here. Delicate horsetail inclusions in demantoid can be damaged by overheating, so material showing intact horsetails has usually not been aggressively treated, though that is an indication rather than proof. Melanite and topazolite reach the market untreated, having little commercial reason to be otherwise.

Where does andradite come from?

The Ural Mountains of Russia are the classic source of demantoid and the reason the variety exists commercially at all. Namibia's Erongo region entered the market in the 1990s, Iran's Kerman province supplies material with horsetail inclusions, and Madagascar produced a substantial find at Antetezambato from 2009.

Away from demantoid, the sources are different. Topazolite is associated with the serpentinites and metamorphic rocks of the Italian Alps, in Val Malenco and the valleys west of Turin. Melanite occurs in the alkaline volcanic districts of central Italy and the Kaiserstuhl in Germany. Iridescent rainbow andradite comes from Nara Prefecture in Japan and from Sonora in Mexico. In the United States the species occurs in California, including in the same San Benito County setting that produced benitoite, and in Arizona. Origin matters commercially only for demantoid, where Russian material carries a premium.

Where the name comes from

Andradite was named in the nineteenth century by James Dwight Dana after José Bonifácio de Andrada e Silva, a Brazilian mineralogist and statesman who had described a related calcium-iron garnet decades earlier. The species name is therefore honorific and says nothing about composition or appearance.

The variety names are more descriptive. Demantoid derives from the German for diamond-like, in reference to the dispersion. Topazolite means topaz-like and refers only to the yellow colour, not to any relationship with topaz, which is a different mineral entirely. Melanite comes from the Greek for black. Allochroite and colophonite are older names for massive or resinous andradite and are effectively obsolete, though they still appear on old museum labels.

What sizes are available?

Faceted andradite is small. Russian demantoid above two carats is uncommon and above five is exceptional; Namibian and Madagascan material runs somewhat larger but clean stones over five carats remain scarce. Topazolite is rarely cut above one carat. Melanite, being opaque, is limited only by the crystal.

The constraint is the rough rather than the demand. Andradite crystallises in skarns, serpentinites and hydrothermal veins where individual clean crystals are small, and much of what is recovered is included, fractured or too dark. Because the price curve for demantoid rises steeply with size, cutters retain weight wherever they can, which is why so many demantoids are cut deep and windowed rather than proportioned for brilliance. That trade-off is worth understanding before paying by the carat.

How much is andradite worth?

Value is decided almost entirely by variety and colour. A fine chromium-green demantoid is among the most expensive garnets in the world, while a yellow topazolite of the same species and the same weight sells as a collector's curiosity, and melanite has little commercial value beyond its use as a black ornamental stone.

Within demantoid the factors in order of weight are colour, then size, then clarity and cut, with origin acting as a modifier. A pure grass green with no brown or yellow cast leads; a brownish or olive stone falls sharply. Clarity expectations are moderate, since the material is typically included, and in Russian stones a well-formed horsetail is treated as desirable rather than as a fault. Cut quality is frequently poor because weight retention dominates, so a stone that is both fine in colour and properly proportioned is rarer than either alone.

Is andradite durable enough to wear?

Andradite is 6.5 to 7 on the Mohs scale with no cleavage and a conchoidal to uneven fracture. That is adequate for earrings, pendants and occasional-wear rings, but it sits at the low end of what withstands daily wear on a hand, and abrasion of facet edges accumulates over years.

Included material deserves more care than the hardness figure suggests. Demantoid with dense horsetails, and any stone with fractures reaching the surface, is more vulnerable to a knock than a clean stone of the same species. Sudden temperature change should be avoided. Warm water, mild soap and a soft brush are sufficient; ultrasonic and steam cleaning are best avoided altogether given how commonly the material is included, and the stone should be removed before any torch work on a setting.

How to buy andradite without being caught out

Establish which variety you are being sold and on what evidence. Demantoid is the only andradite that commands serious money, and a green garnet is not automatically demantoid: chrome tourmaline, tsavorite grossular and peridot all reach the counter under loose descriptions and are separated in seconds on a refractometer, where andradite reads over the limit and they do not.

Then look at the stone against a point light source. The species has more dispersion than diamond, and a green garnet with no fire at all is worth questioning. Examine the pavilion for windowing, which is common because weight is retained at the expense of proportion, and price accordingly. Assume demantoid has been heated unless a report says otherwise, and for any significant purchase buy on a laboratory report from GIA, SSEF or Gübelin, which will also address origin if a Russian premium is being charged. Inclusions are normal; a stone described as flawless demantoid is the claim to doubt. If you want us to look for a particular colour or size, tell us what you are after.

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