Rubies, sapphires, emeralds and diamonds are the four precious stones of the trade. Three criteria classify them: beauty, rarity and durability. Three further elements determine their value: origin, weight and treatments.
What we look at
- Beauty: clarity (the degree of transparency, affected by inclusions), a colour neither too light nor too dark, and brilliance, which depends on the cut.
- Rarity: the scarcer a stone is on the market, the higher its value.
- Durability: hardness, toughness and resistance to chemicals. Hardness is measured on the Mohs scale.
- Origin, weight, treatments: a typical regional colour raises value; weight is given in carats (1 carat = 0.20 g); an untreated stone is worth considerably more.
Ruby
Red corundum, coloured by chromium, with a hardness of 9. The most sought-after come from the Mogok valley in Myanmar, for their intense red that seems to glow from within. Madagascar and Mozambique are important sources today.
Sapphire
Every corundum that is not red is called a sapphire; blue comes from iron and titanium. Myanmar yields deep blues, Sri Lanka bright ones, and Kashmir, historically, the most famous of all. Sapphires also exist in pink, yellow and the rare pink-orange of the padparadscha.
Emerald
Green beryl, coloured by chromium and vanadium, with a hardness of 7.5 to 8. Colombia remains the reference. Emeralds almost always have inclusions, their "jardin". Completely untreated emeralds are extremely rare: oiling is accepted by the trade, and resin filling is the most common treatment. The type and extent of treatment weigh heavily on the price.
Diamond
Pure carbon and the hardest natural material, 10 on the Mohs scale. White diamonds are graded by the 4Cs, with colour from D (colourless) to Z. Synthetic stones with identical properties have been made since the end of the 19th century, so it is essential to check that a diamond is natural.