Ruby, sapphire, emerald: coloured gemstones are not judged like a diamond. Here is what you need to know before choosing, based on the leading laboratories (GIA, SSEF, Gübelin, GRS) and my experience as a diamond dealer.
For a coloured gemstone, colour accounts for most of the value. GIA describes it with three criteria: hue (red, blue, green, sometimes with a secondary tint), saturation (the intensity of the colour) and tone (how light or dark it is). The finest stones have an intense colour, neither too light nor too dark, evenly distributed.
A fine ruby is pure red to slightly purplish red, with no brown. A fine sapphire is a velvety blue, of medium to medium-dark tone. A fine emerald is a vivid, deep green, sometimes slightly bluish.
There is no universal scale like the diamond's 4Cs. Inclusions are expected: almost all emeralds show them to the naked eye, which is called the "jardin" (garden). Some inclusions even add value, such as the fine rutile needles ("silk") that give Kashmir sapphires their velvety look.
Cut matters a great deal: a poorly cut stone lets light escape through the bottom and looks dull in the centre. Finally, large stones of fine quality are rare: the price per carat rises with weight.
Most coloured gemstones are treated. This is not a flaw, provided the treatment is known, stable and clearly stated on the certificate.
A certificate describes the stone as it was at the time of testing: an emerald can be re-oiled afterwards. For an important stone, a recent certificate is preferable.
When they appear on a certificate, these colour names are not just sales talk: the major laboratories reserve them for stones that meet precise criteria.
Each laboratory applies its own criteria: the same term may be granted by one and refused by another.
A gemmological laboratory report states:
Many recognised laboratories analyse coloured gemstones, in Europe as well as in Asia and the United States: GRS, SSEF, Gübelin, GIA, AGL, ALGT, Bellerophon, CGL and many others. Each has its own method and vocabulary; what matters is that the report is clear and verifiable. I explain every certificate to you before any purchase.
For the same quality, yes: an unheated stone is rarer. But a heated sapphire of fine colour remains a valuable natural stone, and heating is stable over time. What matters is that it is stated on the certificate.
That the laboratory found no indication of heating. It is a sought-after mention, especially for fine rubies and sapphires.
Yes, with a few precautions: avoid knocks, take the ring off for sport or DIY, never clean it with ultrasound. For a ring worn every day, a setting that protects the stone is advisable.
Ruby and sapphire, with a hardness of 9 on the Mohs scale. Spinel (8) is also an excellent choice for a ring.
For an important stone, yes. It confirms that the stone is natural, specifies treatments and often the origin. It protects the buyer and makes any resale easier.
Because colour, treatments and origin weigh far more than weight. Two 2-carat sapphires can have very different prices.
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