The Ultimate Guide to Choosing Your Precious Stone: Advice from Bonnot Paris

Each decision narrows the scope of the next. Made in this order, they converge in forty-five minutes; made in reverse, they go in circles for weeks.

We start there because it is the constraint that eliminates options most quickly — and because discussing it openly from the outset avoids three unnecessary appointments.

The case of red deserves to be addressed from the outset, because it often comes as a surprise: a ruby engagement ring starts at more than three times the price of a sapphire ring. This is not a commercial policy — in our stock, the median ruby price per carat is around nine times that of blue sapphire. If the budget is fixed and red is imperative, it is best to know this at the first appointment.

The second question is more decisive than colour: how does the person who will wear this ring live? Manual work, sport, gardening, whether or not the ring is removed for housework.

Above 8 on the Mohs scale — sapphire, ruby, spinel —, no constraints : the ring can be worn for thirty years without a thought. Below that, the setting must protect the stone’s edges, and it should be removed for gardening, sport and housework.

This does not disqualify anyone. An emerald engagement ring can be worn perfectly every day, provided that the bezel setting or low setting is decided at the design stage. The problem is never hardness: it is discovering it afterwards.

This is the decision that matters most and, paradoxically, the quickest: two stones placed side by side under the same light settle in three minutes what no photograph could have resolved in three weeks.

Teal · €1,010/ct median

The colour with no name

Between blue and green, its appearance changes with the lighting. A teal sapphire engagement ring offers a distinctive colour without the premium of classic blue — it is the most abundant family in our stock. 1.65 ct solitaire — 4 680 €.

Blue · €1,780/ct median

The benchmark

The most sought-after colour, and the one whose price rises fastest with weight. A blue sapphire engagement ring is priced partly for its reputation: at comparable quality, it costs nearly twice as much as a teal.

Three points of reference are enough to assess a colour, whatever the family. Saturation drives the price, long before weight. Tone determines readability: too dark, the stone loses its brilliance in the evening; too light, it loses its character. And performance in low light is decisive: a gem that collapses when daylight fades will not leave the drawer.

One misconception to discard: “the darker, the more expensive” is false. A greyish or overly dark blue loses a considerable share of its value, precisely because it loses its brilliance indoors.

The carat is a unit of weight, not size. This is the most costly misconception in the entire process of buying gemstones.

Two practical consequences. First, families cannot be compared in carats : corundum is denser than beryl, so at equal weight a sapphire appears smaller than an emerald. Then, two stones of the same weight can differ by more than one millimetre in diameter depending on their cutting depth — many are cut deep to retain carat weight, and therefore sell for more despite having the same visible surface area.

A few reference points taken from our own stock rather than a theoretical chart: a round stone of approximately 1 ct measures 5.6 to 6.7 mm in diameter; an 1.5 ct oval, from 6.7 × 5.1 to 8.4 × 6.6 mm; a 2 ct cushion, from 6.6 × 5.6 to 8.1 × 6.5 mm.

Almost all gemstones on the market have undergone some form of treatment. The question is therefore not “treated or not”, but which one.

Hence the only truly non-negotiable rule in this guide: an independent laboratory report, verifiable by its number. It says nothing about quality or value — it is a necessary condition, never a sufficient one — but without it, no price makes sense.

It comes last because it adapts to everything that precedes it. The setting is designed around the gemstone’s actual dimensions, never the other way around.

A few useful comparisons. Theoval offers the largest visible surface area for a given weight and elongates the finger. The cushion has presence but appears more compact. Step cuts — rectangular, square — create a mirror effect rather than sparkle, and require impeccable transparency: nothing can be hidden in them.

One question must be asked from the very first appointment, and it is a defining one: “will a wedding band sit against this ring?” If so, the setting height, the band profile and the shape of the basket change immediately. It is the decision people most regret making too late.