Almandine

Garnet group · also called Almandite, Almandine garnet, Carbuncle (historic, for cabochon-cut red garnet)

Almandine is the iron aluminium member of the garnet group and the species behind most dark red garnet ever cut. It is untreated, singly refractive, unusually heavy, and identified in seconds by a three-band iron absorption spectrum. Its central limitation is tone: the colour deepens quickly with size, and bright material above a few carats is uncommon.

What is almandine?

Almandine is the iron aluminium member of the garnet group, the species behind most of the dark red garnet ever cut. Its formula is Fe₃Al₂(SiO₄)₃, its refractive index runs from about 1.78 to 1.83, and its specific gravity of 3.93 to 4.20 makes it one of the heaviest common gemstones.

It is not a stone that appears in nature as a pure end member. Almandine forms a complete solid solution with pyrope, the magnesium aluminium garnet, and a partial one with spessartine, the manganese member, so almost every stone sold under either name is a mixture weighted toward one end. Rhodolite, the purplish red garnet of the Tanzanian and Sri Lankan trade, is simply an intermediate composition on the pyrope–almandine line. The practical consequence is that a laboratory report may describe the same stone as almandine, as pyrope-almandine, or as a garnet of intermediate composition, all correctly.

Almandine is also the most abundant garnet in the earth's crust. It is a common metamorphic mineral in schists and gneisses, which is why it appears as a rock-forming mineral in ordinary geology as well as in the gem trade, and why the industrial abrasive market consumes far more of it than jewellery ever will.

What gives almandine its colour?

Divalent iron in the eight-coordinated site produces the colour, and there is a great deal of it — iron is a required constituent of the formula rather than a trace impurity. That distinguishes almandine from most coloured gems, where a few hundred parts per million of a transition metal is doing the work.

The result is a strong, dark red that shifts toward purple as the pyrope component rises and toward brown as it falls. Because the absorption is so strong, the colour deepens quickly with thickness. A three-millimetre stone can read a clean, bright red while a ten-millimetre stone of exactly the same material reads almost black, and this thickness dependence is the central problem of the species: the material is common, but material pale enough to look good above a few carats is not.

Cutters have addressed this for centuries by removing material from the back. The historic carbuncle — a cabochon-cut red garnet — was routinely hollowed underneath to shorten the light path and lighten the apparent colour, and hollow-backed almandine cabochons are a regular find in antique jewellery. In modern faceting the same problem is handled with shallower pavilions and larger tables.

How is almandine identified?

The absorption spectrum settles it, and it does so with an ordinary hand spectroscope. Almandine shows three strong, broad bands in the green and yellow — near 505, 525 and 575 nanometres — with weaker bands toward the blue and the deep red. This is the classic iron spectrum of gemmology, taught as the first spectrum a student should learn to recognise, and it is present even in stones too dark to judge by eye.

The physical constants confirm it. Specific gravity of 3.93 to 4.20 is high enough to be obvious on a hydrostatic balance, and refractive index of 1.78 to 1.83 means iron-rich material can read at or above the roughly 1.81 limit of a standard refractometer. A red stone that reads over the limit, is singly refractive, and shows the three-band spectrum is almandine and nothing else.

Single refraction excludes ruby, red spinel is singly refractive too but reads 1.712 to 1.736 at a specific gravity near 3.60, and red tourmaline and zircon are both strongly doubly refractive. Many garnets show anomalous double refraction in the polariscope — patchy strain birefringence in a cubic mineral — and this should not be mistaken for genuine birefringence.

The distinguishing feature: stars, and a warning about doublets

Almandine is one of the very few gems that can show a six-rayed star. Oriented needles of rutile, arranged along three directions in the cubic structure, produce four-rayed and six-rayed asterism in cabochon-cut material, and the effect is essentially confined to two places: Emerald Creek in Idaho, where star garnet is the state gem, and the almandine deposits of Odisha in eastern India. No other garnet locality produces it in quantity.

The second thing that belongs specifically to almandine is less welcome. The garnet-topped doublet was the standard cheap imitation of the nineteenth and early twentieth centuries: a thin slice of red almandine fused directly to a body of coloured glass, exploiting the fact that garnet fuses to glass without cement. Because the garnet layer is thin, it colours the finished stone barely at all, so a garnet-topped doublet could imitate a blue sapphire, a green emerald or a yellow stone as easily as a red one.

They remain common in antique jewellery and they are easy to catch once you look. Hold the stone table-down over a white surface and the garnet cap shows as a red patch or a red ring against a differently coloured body. Under magnification the join plane is visible, often with flattened gas bubbles trapped along it, and the two materials take a visibly different polish and lustre. Anyone buying older jewellery should make this check reflexively.

Is almandine treated?

No, and this is one of the species' genuine advantages. There is no accepted heat treatment for almandine, no oiling, no diffusion, no irradiation in commercial use. Garnet in general resists heating because the colour comes from a major constituent rather than from a fragile colour centre, so there is nothing for heat to move.

That means a red garnet is what it appears to be, which cannot be said of most coloured stones at comparable prices. The disclosure question that does arise concerns composites rather than treatments: garnet-topped doublets, and glass or plastic imitations, are the things to exclude.

Some almandine cabochons, particularly star material, are cut with hollow or dished backs to lighten the colour. This is a cutting decision rather than a treatment, but it affects weight and therefore price per carat, and it is worth noticing.

Where does almandine come from?

India is the largest gem source, principally Odisha, Rajasthan and Andhra Pradesh, and Indian material supplies both the cut-stone trade and the star garnet market. Sri Lanka has produced almandine and pyrope-almandine from its gem gravels for as long as records exist, and the Sri Lankan stones tend to be brighter and less iron-saturated.

Madagascar, Brazil, Tanzania and Mozambique all contribute, with African material often falling into the rhodolite range rather than being sold as almandine outright. In the United States, Emerald Creek in Idaho is the source of six-rayed star garnet and one of the few gem deposits open to public digging, while Gore Mountain in New York produces almandine crystals of extraordinary size — up to a foot across — that are almost entirely opaque and were long mined as abrasive rather than as gem material. Austria's Zillertal is a classic European occurrence.

Industrially, almandine dominates. The garnet used in abrasive blasting and waterjet cutting is largely almandine, mined at very large scale, and this trade dwarfs the gemstone market in tonnage by orders of magnitude.

Where the name comes from

The name descends from Alabanda, a city in Caria in south-western Asia Minor, recorded by Pliny as a centre where red stones were cut and traded. Alabandicus became alabandine and then almandine, a corruption that has been in place for centuries and is now simply the accepted mineral name.

The stone itself has an older commercial history than the name. Garnet cut into thin plates and set over gold foil is characteristic of Anglo-Saxon, Frankish and Gothic metalwork from the fifth to the eighth centuries — the Sutton Hoo material is the best-known example — and trace-element work has indicated that much of that garnet travelled from India and Sri Lanka into northern Europe. It is one of the longest-documented gemstone trade routes in existence.

What sizes are available?

Large, and this is not a constraint. Almandine rough of clean gem quality in tens of carats is readily available and inexpensive, and faceted stones of twenty, fifty and a hundred carats exist without being remarkable.

The limit is optical rather than physical. Above roughly five carats the depth of colour usually pushes the stone toward black in ordinary lighting, so the useful size range for a stone that still looks red is much narrower than the material would allow. Pale, bright almandine and pyrope-almandine of ten carats and up is genuinely uncommon and is priced accordingly, which produces the odd situation of a cheap material with an expensive good example.

How much is almandine worth?

Tone comes first, and by a wide margin. The question that decides the price of a red garnet is whether it is still red at size, and a bright, medium-toned stone of five carats is worth many times a dark one of the same weight. Everything else follows behind.

Clarity comes second, and here almandine is helped by its abundance: eye-clean material is normal, so included stones sell at a real discount rather than being tolerated. Cut comes third and matters more than in most inexpensive stones, because good proportions are the cutter's only tool against excessive depth of colour. Size matters least, and above about five carats the per-carat price of ordinary dark material stops rising at all.

Star garnet is priced on a separate basis, on the sharpness, centring and completeness of the star, with six-rayed stones well above four-rayed. Compared with the rarer garnets — demantoid and tsavorite — almandine is inexpensive by two or three orders of magnitude, and the gap is a matter of scarcity rather than of any defect in the material.

Is almandine durable enough to wear?

Yes, and better than its price suggests. At 7 to 7.5 on the Mohs scale almandine is harder than airborne quartz dust, so it holds a polish through daily wear, and it has no cleavage at all, so it will not part under a knock the way topaz or kunzite can.

The realistic risk is fracture from a hard impact along an existing inclusion, which is a concern in heavily included material and rarely otherwise. Garnets are stable to light and to ordinary chemicals, and they do not fade.

Heat is the one caution. Sudden temperature change can fracture a stone with liquid or crystal inclusions, so a garnet should be removed before any repair work involving a torch. Warm water, mild soap and a soft brush are the right routine; ultrasonic cleaning is generally safe on clean material but should be avoided on anything visibly included, and on antique pieces where a doublet or a foil backing may be present.

How to buy almandine without being caught out

Judge the stone in ordinary indoor light, not under a spotlight. Garnet dealers light their stock from directly above and from close range, which lifts a dark stone into apparent brightness that it will not have on a hand across a table. If it is not red in the palm of your hand in a normally lit room, it is not a red stone.

Turn any antique piece face-down and look for the red ring of a garnet-topped doublet. This takes a moment and catches an imitation that is still widely present in the second-hand market.

Ask which garnet it is, and do not be troubled by an answer of pyrope-almandine or rhodolite rather than almandine — these are compositional descriptions on a continuous series, and a laboratory will use whichever is accurate. Be more troubled by a seller who cannot say.

Finally, price cut and tone rather than weight. In a material this abundant, carat weight is the least informative number on the ticket. See our gems, or tell us what you are looking for.

Looking to buy almandine? See the collection, the rough, or send us a brief.