Garnet: The Family, Properties, Origins and Value

Garnet is a word that covers a family, and the family covers most of the colour wheel. Six end members share one crystal structure and swap elements freely, so that most garnets in the trade are mixtures rather than pure species, and the mixtures have names of their own. Red almandine in a Victorian brooch, green demantoid in a Russian tiara, orange mandarin from Namibia and green tsavorite from Kenya are all garnet, and the word tells you almost nothing about the price. The theory does.

What it is

The general formula is X₃Y₂(SiO₄)₃, where X is a large cation, magnesium, iron, manganese or calcium, and Y a smaller one, aluminium, iron or chromium. Fill the sites one way and you have pyrope, magnesium aluminium garnet; another and almandine, iron aluminium; another and spessartine, manganese aluminium. The calcium garnets are grossular, with aluminium, andradite, with iron, and uvarovite, with chromium. All crystallise in the cubic system, usually as dodecahedra, and all are singly refractive with no pleochroism, so the colour reads the same from every angle. Hardness runs from 6.5 to 7.5 by species, refractive index from about 1.71 in grossular and pyrope to nearly 1.89 in andradite, and specific gravity from 3.5 to 4.3. No cleavage and good toughness across the family. Because the species mix freely, gemmologists identify a garnet by measuring where it sits on the scales rather than by naming a pure species, and most stones sit between two.

Where the colours come from

Iron, Fe²⁺, gives almandine and the pyrope-almandine mixtures their red, and in the East African pyrope-almandine called rhodolite a purplish raspberry red that is the family’s most familiar fine colour. Pure pyrope would be colourless; with chromium and iron it is the dark red of Bohemian garnet. Manganese gives spessartine its intrinsic orange. Vanadium and chromium give grossular the green of tsavorite; iron gives it the orange-brown of hessonite; the calcium chromium garnet uvarovite is green throughout and almost never cut. Andradite is coloured by iron, and with chromium becomes demantoid, a green with a dispersion higher than diamond’s, which is what the name means. Vanadium in pyrope-spessartine gives the colour-change garnets of Madagascar and East Africa, bluish green by day and purplish red by lamp. Mixtures of grossular and andradite give the yellow-green stones from Mali. There is no blue garnet in any fixed light.

Where it comes from

Everywhere, which is part of why the word carries no premium. The fine members are specific. Demantoid from the Urals in Russia, mined since the 1850s, carries the horsetail inclusions of chrysotile fibres that the market accepts as a signature of origin and pays for; Namibia, Madagascar, Iran and Italy produce demantoid without them. Tsavorite comes from one belt in Kenya and Tanzania and has its own note. Spessartite’s mandarin material comes from Namibia, Nigeria and Tanzania and has another. Rhodolite is East African and Sri Lankan; malaya, the pyrope-spessartine in pink-orange, comes from the Umba valley on the Kenya-Tanzania border; colour-change garnet from Bekily in Madagascar and from Tanzania. Hessonite is Sri Lankan and Indian. Commercial red almandine and pyrope come from India, Brazil, Madagascar, Mozambique and a dozen other countries and are priced as material.

How it is judged

By species first, because the species sets the price range, and then by colour on the three axes. In the reds the fault is tone: almandine and pyrope run dark and a stone that reads black in a room is worth little whatever its saturation, so rhodolite’s lighter purplish red commands the premium. In demantoid it is the balance between a saturated green and the dispersion that dark tone swallows, and the horsetail, which in Russian stones adds rather than subtracts. In tsavorite and spessartite it is purity of hue without olive or brown. Colour-change garnet is judged on the strength and cleanness of the change, as alexandrite is. Then clarity, where the family is generally clean except spessartite and demantoid; then cut, which should use the high refractive indices rather than hide weight; then size, where demantoid and tsavorite step steeply above a few carats and the commercial reds barely reward it at all.

Treatments, and what to demand on paper

Almost none exist. No commercial heat, diffusion or filling improves any garnet, and the family is sold as the ground made it, which is rare in this trade and worth saying plainly to a buyer arriving from sapphire. The one exception is demantoid, which is sometimes heated to reduce a yellowish component, and a report should say so. The paper’s job is identity: separating the species from each other, from glass, and from the synthetic garnets of other chemistries that were once grown as diamond imitations. For the fine members, tsavorite, demantoid and mandarin, a report from a serious house on any stone of size is reasonable; for commercial garnet it is not needed.

The market

The widest range under one name. Commercial red garnet is priced by the carat as material. Rhodolite of fine colour and size is fine jewellery material. Malaya and colour-change garnet are collectors’ stones with small markets. At the top, Russian demantoid with horsetails and fine tsavorite above five carats trade per carat at levels that surprise buyers who know the family only by its bottom, and both have appreciated steadily this century as buyers learned to price the species rather than the word. Liquidity is thin at the top and broad at the bottom, and no member of the family has an auction record book or a colour appellation.

Care

Hardness 6.5 to 7.5 and no cleavage make the family sound for daily wear, with the softer species, demantoid and pyrope, abrading faster in rings. Ultrasonic cleaning is safe on clean stones; demantoid with horsetails and heavily included spessartite should stay out of it. Nothing in the family is filled, oiled or heat-sensitive.

Where I stand

The garnet family is the most undervalued in the trade, and I hold its finest member, tsavorite, in this office’s portfolio. I recommend demantoid to collectors who want the family’s rarest stone and mandarin spessartite to anyone who wants an orange, and I treat the rest as what it is, honest jewellery material that needs no hunter. The rule is the one that applies to every stone with a family name attached: buy the species, not the word.

Questions collectors ask

What is the most valuable garnet?

Demantoid from the Urals with the horsetail inclusions the market prizes, and tsavorite of fine green above a few carats, run close together at the top, followed by mandarin spessartite and fine colour-change garnet. Commercial red almandine and pyrope sit at the bottom of the range. The word garnet covers a range of prices wider than almost any other in the trade.

Are garnets treated?

Almost never. No commercial heat, diffusion or filling exists for the family, because nothing in the standard repertoire improves it. Demantoid is occasionally heated to shift its colour and that should be disclosed. Otherwise a garnet is sold as the ground made it, which is unusual in this trade.

Is there a blue garnet?

Only as a colour-change stone: pyrope-spessartine garnet from Madagascar and East Africa coloured by vanadium can read bluish green or greyish blue in daylight and purple or red under a lamp. A garnet that is plain blue in all light does not exist in nature, and anyone selling one is selling something else.