Carat, Clarity, Brilliance: The Vocabulary of Gemstones Decoded
Carat, cut, clarity, lustre, brilliance, fire… The vocabulary of precious stones can feel impenetrable. Here is the essential lexicon, explained simply — so you can buy while understanding what you are being told.
The carat: a weight, not a size
First misunderstanding to clear up: the carat measures weight (1 ct = 0.20 grams), not dimensions. Two one-carat stones can look very different in size depending on their density and proportions: a sapphire, denser than a diamond, looks smaller at equal weight. Hence the importance of always checking the dimensions in millimetres on the listing — those are what your eye will see.
Colour: hue, saturation, tone
Professionals describe a colour along three axes: hue (blue, blue-green…), saturation (intensity, from dull to vivid) and tone (lightness, from pale to dark). The ideal stone? Strong saturation, medium tone: a vivid colour that stays luminous. This is the number one criterion of value in a coloured stone.
Clarity: “eye clean”, the term to know
Clarity describes the presence of inclusions. For coloured stones, the practical standard is “eye clean”: no inclusion visible to the naked eye at normal distance. There is no point hunting for perfection under a microscope — unlike diamond, coloured stones are judged by eye, and some species (emerald first among them) almost always carry natural inclusions.
Lustre, brilliance, fire: light in three words
- Lustre: the light reflected by the polished surface;
- Brilliance: the light returned from inside the stone to the eye;
- Fire: the splitting of light into rainbows (strong in diamond and zircon).
A “well-cut” stone maximises brilliance; a “windowed” stone lets light pass straight through without returning it — a classic cutting fault to avoid.
And the famous “origin”?
Ceylon, Burma, Colombia, Mozambique… Geographical origin, determined by laboratories, influences prestige and price — we will devote an article to it. In the meantime, put your new vocabulary to work on our latest stones, detailed listings in hand, or come and handle the gems in Paris — the best vocabulary lesson there is.
