Demantoid

Demantoid is the garnet that behaves like a diamond. Its dispersion, the property that splits white light into spectral colours, is higher than diamond's, so a well-cut demantoid throws flashes of red and orange out of a green body in a way no other green stone can. The name comes from an old word for diamond, and it was given for exactly that reason.

It is also, by some distance, the rarest and most expensive garnet, and the one whose price depends most on where it came from. A fine Russian stone with the right inclusions can cost ten times what an equally green stone from elsewhere costs, and the reason is a feature that in any other gem would be a flaw.

This page covers what the fire is and why it matters more here than anywhere else in the garnet family, the horsetail inclusions and why they raise the price rather than lowering it, the sources, the size problem, and how it compares with the other green garnet it is regularly sold alongside.

What it is, and where the fire comes from

Demantoid is the green variety of andradite, a calcium iron garnet. The fine green comes from traces of chromium, the same element that colours emerald; where chromium is absent and only iron is present, the colour drifts towards a yellowish green that the trade values considerably less.

Two optical numbers explain the stone. Its refractive index is around 1.89, which is close to diamond's and gives it a hard, bright lustre. Its dispersion is 0.057 against diamond's 0.044, which is what produces the coloured fire. No other garnet comes close on either figure, and very few gems of any kind do.

The fire is only visible when the cut lets it out, and it is easily swamped by a strong body colour. This is the unusual trade-off in demantoid: the deepest green stones show the least fire, and the liveliest stones are a slightly lighter green. Neither is wrong, but a buyer should decide which they are paying for.

Horsetails, and why a flaw raises the price

Russian demantoid characteristically contains inclusions of fine golden fibres of chrysotile radiating from a point, like the tail of a horse. In any other stone these would be a clarity defect. Here they are a certificate of origin, because the Ural deposits are the only ones that reliably produce them, and the Urals are where the stone's reputation was made.

A stone with a well-formed, centred horsetail visible under a loupe commands a premium over an equally green clean stone without one. That is a market convention, not a statement about beauty, but it is a firm one, and a report that describes horsetail inclusions is worth having in writing.

Namibian and Madagascan material rarely shows them. It can be a fine, bright green in its own right and is priced accordingly, but it is priced as demantoid from those sources rather than as Russian demantoid, and the gap between the two is real.

Judging a demantoid

Colour and fire pull against each other, and the market has settled on a middle: a vivid green that still shows fire is the top of the range. A very dark stone will be cheaper than its green alone would suggest, because it has given up the one thing that makes demantoid demantoid.

Demantoid against tsavorite

Both are green garnets and they are routinely offered under that phrase, which hides the fact that they are different species with different virtues. Tsavorite is grossular: harder at 7 to 7.5, tougher, typically cleaner, with a purer green and almost no fire. Demantoid is andradite: softer at 6.5 to 7, more included, with the coloured fire that tsavorite cannot produce.

If you want a durable green stone for daily wear, our tsavorite guide is the better read. If you want the most brilliant green stone that exists, this is the page. The two are complementary rather than competing, and a dealer who blurs them is either careless or hoping you are.

The size problem

Demantoid crystals are small. The overwhelming majority of cut stones are under a carat, a clean two carat stone is a serious find, and anything above five carats belongs in a museum or an auction catalogue. Russian material in particular rarely exceeds a carat, which is one reason fine Russian stones of any size command what they do.

Price per carat therefore rises very steeply with weight, and comparing quotes across sizes is meaningless. Compare like with like, and expect a two carat stone to cost far more per carat than two one carat stones of the same quality.

Where it comes from

The Ural Mountains, from 1868, are the classic source and the one that made the stone. Demantoid was a favourite of the Russian court and of Fabergé, and most of the antique material in circulation is Russian. The deposits were largely idle through the Soviet period and have been worked again, in small quantities, since the 1990s.

Namibia has been the most important modern source since the Green Dragon mine opened in the Erongo region in the 1990s, producing bright stones that are generally free of horsetails. Madagascar produced a notable find in 2009, and Iran and northern Italy both contribute in small amounts. Origin matters here in a way it does not for most coloured stones, and it should be on the report.

Treatment

Most demantoid is untreated, and Namibian material essentially always is. Some Russian stones are gently heated to reduce a brownish component and improve the green. The heating is low-temperature, stable and difficult to detect, and it is not universally disclosed.

For a stone at demantoid prices, that is worth asking about directly and worth having a laboratory comment on. An unheated Russian stone with horsetails is the top of this market, and the word unheated should not be taken on trust.

Hardness and everyday wear

At 6.5 to 7 on the Mohs scale with no cleavage, demantoid is a workable ring stone, though softer than tsavorite and noticeably softer than sapphire. The facet edges will scuff over years of hard wear and the polish will dull, both of which are repairable.

The greater caution is the inclusions. A horsetail is a bundle of fibres, and ultrasonic and steam cleaning can both open a fracture along it. Warm water and a soft brush are enough. Our guide to settings covers mounting; a setting that protects the girdle is sensible for a stone this valuable, and a low profile keeps the fire visible without exposing the edges.

What it costs

Origin and horsetails first, colour and fire second, size third. In dollars per carat:

Those top figures sit alongside fine ruby and well above tsavorite, and they reflect a market driven by collectors and by the antique trade as much as by jewellery buyers. This is not a stone to buy for the name alone; the name is doing a great deal of the pricing.

Meaning and symbolism

Demantoid is a garnet, so it carries the January birthstone with the rest of the family, and the January page covers the group. Our garnet guide covers the red stones most people picture when they hear the word.

Garnet tradition runs to constancy, safe return and steady commitment across most of the cultures that used it. Demantoid's own history is short and specific: it was the stone of late imperial Russia, and much of the symbolism attached to it today is really the glamour of that period.

Designing around a demantoid

• Let the light in. The fire is the point, and a bezel that shades the crown or a deep setting that buries the pavilion wastes it. An open gallery and a moderate height are right here.
• Protect the girdle even so. A half-bezel or a well-made prong setting with the edges covered is the sensible compromise for a stone at this price. Our settings guide covers the options.
• Yellow gold suits the green and echoes the Russian jewellery the stone is associated with; white metal makes the fire read cooler. Our metals guide covers the trade-off.
• Keep the accents small. Demantoid's brilliance holds its own against diamond, but a halo of larger stones competes with fire that is subtle by nature.
• Decide colour or fire first. A very dark stone and a lively one are both demantoid; they are different purchases, and the design should serve whichever you chose.

A short buying checklist

• Confirm the species in writing: andradite garnet, demantoid, not simply green garnet.
• Ask the origin and expect it on the laboratory report, since Russian and Namibian material are priced differently.
• Ask for the horsetail to be shown under magnification, and ask whether it is described on the report.
• Ask directly whether the stone has been heated; the answer is not always volunteered.
• Judge fire by tilting under a single light, and judge colour separately in daylight.
• Compare quotes only at the same carat weight, since price per carat climbs steeply with size.

Frequently asked questions

Is demantoid more brilliant than diamond? Its dispersion is higher, so in the right cut it shows more coloured fire. Its lustre is slightly lower and it is far softer, so it does not keep that brilliance under hard wear the way diamond does.

Why do inclusions make it more expensive? Because horsetail inclusions are found almost only in Russian material, and Russian demantoid is what the market prizes. They are proof of origin, and the market pays for origin here.

Is demantoid treated? Usually not, and Namibian material essentially never. Some Russian stones are gently heated to improve the green, and that is not always disclosed. Ask, and have a laboratory comment.

Is it hard enough for an engagement ring? It is usable at 6.5 to 7 with no cleavage, but it is softer than tsavorite or sapphire, and its inclusions dislike ultrasonic cleaning. A protective setting and some care are needed.

What is the difference between demantoid and tsavorite? Different garnet species. Tsavorite is harder, cleaner and purer green; demantoid is softer, more included and has the fire tsavorite cannot show.

Why is it so hard to find a large one? The crystals form small, and clean large ones are genuinely rare. Anything over two carats is a specialist purchase and over five is a museum stone.

Does origin really matter? More than for almost any other coloured stone. Russian demantoid with horsetails commands a large premium over material of the same colour from Namibia or Madagascar.

How do I clean it? Warm water, mild soap and a soft brush. Avoid ultrasonic and steam cleaners, which can open a fracture along the fibrous inclusions the stone is valued for.

To see how it sits against the rest of the coloured stones, the semi-precious stones guide compares the series side by side, and we are happy to answer questions directly (opens in new tab).