Ruby: Properties, Origins, Treatments and Value

Ruby is red corundum. That sentence contains most of the theory, because corundum is aluminium oxide, one of the hardest and most stable minerals in the crust, and red is what happens when a small amount of chromium takes the place of aluminium in its lattice. The chromium strains the structure, encourages fractures, and rarely meets aluminium in quantity in the same rock. Large, clean, well-coloured ruby is therefore rare in a way an equivalent sapphire is not, and everything the trade does around the stone, the heating, the filling, the appellations, the confident origin stories, follows from that scarcity.

What it is

Corundum is Al₂O₃, crystallising in the trigonal system, usually as barrel-shaped or tabular hexagonal prisms. Hardness 9 on the Mohs scale, second only to diamond, with no cleavage and good toughness, which is why ruby survives centuries of wear. Refractive index 1.762 to 1.770, birefringence 0.008, specific gravity close to 4.00. It is pleochroic: purplish red down one direction of the crystal and orangey red down the other, and a cutter orients the table to show the purplish red face-up, because that is the colour the market pays for. Red corundum is ruby; every other colour of corundum, including pink, is sapphire, and the line between pink sapphire and ruby is drawn by laboratories rather than by nature.

Where the colour comes from

Chromium, Cr³⁺, substituting for aluminium. It absorbs in the violet and the yellow-green and transmits red, and at good concentration in a low-iron host it does something more: it fluoresces, absorbing visible light and re-emitting it as deep red, so the stone adds light of its own to the light falling on it. Iron quenches that fluorescence. This is why the host rock decides the character of a ruby. Marble-hosted deposits are poor in iron and produce the glowing red the trade calls Burmese; amphibolite and basalt-related deposits carry iron and produce cleaner, quieter, often larger stones that return only the light they are given. The difference between Burma and Mozambique is, at bottom, a difference in iron.

Where it comes from

Burma remains the reference. The Mogok Stone Tract in upper Burma has produced the finest ruby on record for as long as there have been records, from marble, with intact rutile silk and strong fluorescence. Mong Hsu, further east, is marble-hosted too but yields stones with a blue core that heat removes, and nearly all of it is treated. Mozambique came into production at Montepuez around 2009 and now supplies a large share of the fine ruby on the market from amphibolite. Tanzania gives varied material from Winza and Songea; Vietnam from Luc Yen; Afghanistan from Jegdalek; Tajikistan, Madagascar, Kenya, Malawi and Greenland each contribute. Thailand and Cambodia, once major sources of dark iron-rich ruby, are now better known as the places where the world’s ruby is heated and cut. Sri Lanka produces mostly pink corundum that the laboratories often decline to call ruby at all.

Origin is an opinion. A laboratory forms it by matching inclusions, trace-element chemistry and spectral behaviour against reference stones, and marble-hosted deposits overlap closely enough that some stones are never resolved. Where the origin premium is doing serious work in the price, a second report is normal practice.

How it is judged

Colour first, on three axes. Hue: red, with a slight purplish lean permitted and often welcome, orange and brown not. Saturation: it has to hold, so the red does not wash out toward the girdle or go grey. Tone: dark enough to read red, not so dark that the stone closes up at dinner. The three axes are what a laboratory describes and what the price follows, and saturation moves it most. At the top of the scale sits the appellation pigeon’s blood, granted by some houses and not others, against criteria that do not agree.

Then the face. Ruby reaches the market in native cuts that keep weight at the expense of light: deep pavilions, windows, extinction across the crown. Colour seen through the side of such a stone is not the colour a setting will show. Then clarity, judged face-up rather than under the loupe, because silk and fine inclusions are normal in ruby and intact silk is itself evidence that the stone has not been heated. Then weight, and the price per carat curve is steeper for ruby than for any other stone, because fine ruby above three carats is countable and above ten carats is historic.

Treatments, and what to demand on paper

Heat is the oldest: it dissolves silk, improves apparent clarity and moves colour, and disclosed it is honest work. Flux-assisted heating heals fractures and leaves glassy residues, which a report states separately. Lead-glass filling turns opaque low-grade corundum into a bright red composite that is not a treated ruby but a manufactured object, damaged by a jeweller’s torch and by household acids. Diffusion and dyeing surface from time to time. Synthetic ruby has been grown since 1902 and still arrives in parcels with stories attached. A report from Gübelin, SSEF, GIA or AGL addresses all of this in order: identity, treatment, origin, and only then a colour name. No indications of heating is the phrase to want, and the traps are all failures to read that line.

The market

Ruby sets the records for coloured stones. The Sunrise Ruby, a 25.59-carat Burmese stone, sold in Geneva in 2015 for around thirty million dollars, roughly 1.2 million a carat; the Estrela de Fura, a 55.22-carat Mozambican stone, sold in New York in 2023 for a reported 34.8 million, the highest total for any ruby. What those records reward is size, saturation, the unheated line and origin, in that order. Below the records the same four variables set every price, and the unheated premium has been the fastest-appreciating part of it for thirty years, because certified unheated material shrinks every season while the demand for proof grows.

Care

Hardness 9 and no cleavage make ruby the most wearable of the great stones: it takes daily wear, ultrasonic cleaning and a jeweller’s torch without complaint. The exceptions are the treated ones. Glass-filled composites are damaged by heat, acids and even lemon juice; heavily fractured stones of any kind deserve a protective setting. An unheated ruby with intact silk will outlast its keeper and the keeper after that.

Where I stand

Ruby is the stone this office was built on. I buy it unheated only, at origin where I can and in Bangkok where the parcels come to me, in daylight against master stones, and I send every stone to a Swiss laboratory before a client hears of it. Burma first, Mozambique when the colour is there, and nothing that has seen a furnace. The buyer’s guide sets out what that means stone by stone. The theory above is what it rests on.

Questions collectors ask

What is the difference between ruby and sapphire?

None in mineralogy. Both are corundum, aluminium oxide. Red corundum coloured by chromium is called ruby; every other colour, including pink, is called sapphire. Where red ends and pink begins is a laboratory judgement, and it moves the price by a multiple.

Which ruby origin is the most valuable?

Burma, and within Burma the Mogok Stone Tract, because its marble-hosted ruby is low in iron and fluoresces in daylight. Mozambique now supplies much of the fine ruby on the market and the best of it is superb, but for comparable stones Burmese origin carries a durable premium confirmed by every auction record.

Is heated ruby worth buying?

As a stone to wear, a fine heated ruby honestly disclosed is a legitimate purchase at a lower price. As an asset, no: the unheated premium is the part of ruby prices that has grown fastest for thirty years, and the resale market for serious stones asks for the unheated line on Swiss paper first.