Precious gemstones guide
A short guide to the stones we know best: what makes them precious, where the finest come from, and what to look for before you buy.
What makes a stone precious
Three criteria classify a precious stone. Three further elements then determine its value. This is what we teach in our seminars and apply in every appraisal.
The three criteria
- Beauty: a colour neither too light nor too dark, good transparency, and the brilliance given by a well-proportioned cut.
- Rarity: the fewer comparable stones exist on the market, the higher the value.
- Durability: hardness, toughness and resistance to heat and chemicals, so the stone can be worn for generations.
What determines the value
- Origin: some regions give a colour so typical that the origin alone adds a premium, such as Mogok for rubies or Kashmir for sapphires.
- Weight: expressed in carats (1 carat = 0.20 g). Above certain sizes, fine stones become exponentially rarer, and therefore exponentially more valuable.
- Treatments: most stones on the market are treated; it is estimated that fewer than 5% of rubies and sapphires have never been heated. An untreated stone of the same appearance is worth considerably more.
The Big Four, stone by stone
Rubies, sapphires, emeralds and diamonds have been the reference precious stones for centuries.
Ruby
The red variety of corundum, coloured by chromium. Chromium also makes rubies fluoresce under ultraviolet light, which is why the finest seem to glow from within. The most celebrated come from the Mogok valley in Myanmar; Mozambique has become the main source of fine rubies over the last fifteen years. It is estimated that fewer than 5% of the rubies on the market have never been heated.
- Mineral
- Corundum
- Hardness
- 9 on the Mohs scale
- Finest origins
- Myanmar (Mogok), Mozambique, Madagascar
- Common treatments
- Heating; lead-glass filling for low qualities
- What to look for
- A pure, intense red without brown or too much purple, lively in daylight
Sapphire
Every corundum that is not red is called a sapphire. Blue comes from iron and titanium. Kashmir sapphires, mined mostly in the 1880s, are famous for their soft, velvety blue; Myanmar yields deep blues, Sri Lanka bright and luminous ones, and Madagascar has produced remarkable stones since the 1990s. As with rubies, fewer than 5% of the sapphires on the market are estimated never to have been heated. Sapphires also exist in pink, yellow, orange, purple and green.
- Mineral
- Corundum
- Hardness
- 9 on the Mohs scale
- Finest origins
- Kashmir, Myanmar, Sri Lanka, Madagascar
- Common treatments
- Heating; beryllium diffusion
- What to look for
- A saturated blue that stays blue in every light, evenly spread, without grey or dark zones
Emerald
The green variety of beryl, coloured by chromium and vanadium. Emeralds almost always contain inclusions, called their "jardin" (garden), and a clean emerald is exceptionally rare. Completely untreated emeralds are extremely rare too: oiling is accepted by the trade, and resin filling is the most common treatment. Colombia remains the reference, with mines such as Muzo and Chivor; Zambia gives emeralds with a slightly bluer, cooler green.
- Mineral
- Beryl
- Hardness
- 7.5 to 8 on the Mohs scale, but more fragile because of its inclusions
- Finest origins
- Colombia, Zambia, Afghanistan (Panjshir)
- Common treatments
- Filling of fissures with oil or resin to improve clarity
- What to look for
- A vivid, pure green, good transparency, and as little filling as possible
Diamond
Pure carbon, crystallised deep in the earth, and the hardest natural material. White diamonds are graded by the 4Cs: carat, colour (from D, colourless, to Z), clarity and cut. Natural coloured diamonds, such as yellow, pink or blue, form a separate and much rarer market.
- Mineral
- Carbon
- Hardness
- 10 on the Mohs scale
- Major sources
- Botswana, Russia, Canada, South Africa
- Common treatments
- Laser drilling, fracture filling, high-pressure high-temperature (HPHT) colour treatment
- What to look for
- A laboratory certificate, an excellent cut, and confirmation that the diamond is natural
Origins and prices
Two stones of the same size, colour and quality can differ in price several times over because of where they were mined. For rubies, sapphires and emeralds, origin is one of the first things a buyer asks about. For white diamonds, it hardly changes the price.
| Stone | Most valued origins | Other important sources | How much origin matters |
|---|---|---|---|
| Ruby | Myanmar (Mogok) | Mozambique, Madagascar, Thailand, Sri Lanka | A great deal. A fine unheated Burmese ruby is among the most expensive coloured stones. Mozambique rubies have narrowed the gap since 2009, but at the same quality they usually sell for less. |
| Sapphire | Kashmir, then Myanmar | Sri Lanka, Madagascar | A great deal. In our analysis, a Kashmir sapphire sells for about ten times a comparable stone from Sri Lanka, and a Burmese one for about 60% more. |
| Emerald | Colombia (Muzo, Chivor) | Zambia, Afghanistan (Panjshir), Brazil | A lot. Colombian emeralds carry a premium over Zambian stones of similar quality. The amount of oil in the fissures weighs as much as the origin. |
| Diamond | No origin premium for white diamonds | Botswana, Russia, Canada, South Africa | Very little. White diamonds are priced on the 4Cs, with the Rapaport list as the trade reference; origin matters for traceability and sanctions. The exceptions are historic Golconda diamonds from India and pink diamonds from Argyle in Australia, a mine that closed in 2020. |
Sapphires: what origin does to the price
Comparable blue sapphires, all else equal, Sri Lanka = 100
Between the four stones
Per carat, the most expensive stones of all are natural coloured diamonds, above all blue and pink. Next come the finest Burmese rubies, which over 5 carats are rarer than diamonds of the same size. Fine emeralds and sapphires follow. These public auction records give an idea of the top of each market.
| Stone | Weight and origin | Sale | Price | Per carat |
|---|---|---|---|---|
| Oppenheimer Blue diamond | 14.62 ct, fancy vivid blue, South Africa | Christie's Geneva, 2016 | USD 57.5 million | ≈ USD 3.9 million |
| Sunrise Ruby | 25.59 ct, Myanmar | Sotheby's Geneva, 2015 | USD 30.3 million | ≈ USD 1.2 million |
| Estrela de Fura ruby | 55.22 ct, Mozambique | Sotheby's New York, 2023 | USD 34.8 million | ≈ USD 630,000 |
| Rockefeller Emerald | 18.04 ct, Colombia | Christie's New York, 2017 | USD 5.5 million | ≈ USD 305,000 |
| Blue Belle of Asia sapphire | 392.52 ct, Sri Lanka | Christie's Geneva, 2014 | USD 17.3 million | ≈ USD 44,000 |
Prices rounded, in US dollars at the time of sale, including buyer's premium. Records show the very top of the market; most fine stones sell for a small fraction of these amounts.
Imitations, synthetics and treatments
A red stone that looks like a ruby can be a natural ruby, a treated ruby, a ruby grown in a laboratory, or red glass. To the eye they can look much the same, but they are worth very different amounts.
How a laboratory classifies a stone
What the same blue sapphire is worth
Indicative value of stones of similar appearance, natural untreated = 100
Imitations
An imitation only looks like the gem. It can be glass, plastic or a cheaper natural stone: red glass or garnet for ruby, blue glass for sapphire, cubic zirconia for diamond. Doublets and triplets glue a thin slice of real stone onto glass or a lesser material, so that the top looks genuine.
How it is detected: its physical and optical properties are different. A trained gemmologist with a loupe, a refractometer and a microscope can usually identify it.
Synthetics
A synthetic stone has the same chemistry, crystal structure and optical properties as the natural gem, but was grown in a laboratory in a few weeks instead of forming over millions of years. The first synthetic rubies, the so-called "Geneva rubies", appeared around 1885; Auguste Verneuil's flame-fusion process followed in 1902. Today rubies, sapphires and emeralds are also grown by flux and hydrothermal methods, and diamonds by CVD and HPHT.
How it is detected: curved growth lines, gas bubbles or flux residues for older methods; for modern synthetics, often only the instruments of a gemmological laboratory. They must be sold as "synthetic", "laboratory-grown" or "laboratory-created".
Treatments
A treated stone is natural but was altered after mining to improve its colour or clarity. Some treatments are traditional and accepted, like heating rubies and sapphires or oiling emeralds. Others turn low-quality material into something that only looks fine, like lead-glass filling or beryllium diffusion.
How it is detected: changes to inclusions under the microscope, spectroscopy and chemical analysis. A laboratory certificate states it, for example "no indications of heating" or "minor" to "significant" clarity enhancement.
Every treatment must be disclosed at sale. We only sell natural, untreated precious stones, appraised by us and certified by third-party, internationally recognised laboratories.
Origin and laboratory certificates
For coloured stones, leading laboratories such as SSEF, Gübelin and GRS identify the stone, detect treatments and give an opinion on geographic origin. Some also use trade names such as "pigeon's blood" for rubies or "royal blue" for sapphires, but only for stones that meet strict criteria of colour and saturation.
A certificate confirms what a stone is. It never gives a price, and it does not say whether a stone is beautiful: that judgement takes the experienced eye of someone who knows the market.