Fancy coloured diamonds

White diamonds are graded on the GIA scale from D (colourless) to Z (light yellow, brown or grey), described in the diamond colour entry. A fancy colour diamond is one that falls outside that range. GIA gives two ways to qualify: the stone shows more colour than a Z reference stone, or its colour is something other than yellow, brown or grey. A pale yellow diamond that sits within D to Z is therefore not a fancy yellow; a strongly yellow one beyond Z is. A pink or blue stone is fancy at any strength.

GIA also cautions that there are no universal rules for using the word "fancy", so the label is most reliable when a recognised lab report backs it. A grading report is worth even more for a fancy colour diamond than for a white one.

Most colours come from something that interrupted the crystal as it formed. Yellow diamonds get their colour from trace amounts of nitrogen, and when the nitrogen atoms are spread evenly the yellow looks strong and vibrant, while clustered nitrogen gives softer, lighter hues. Blue diamonds get theirs mainly from boron. Pink diamonds are different: their colour is thought to come from stress in the crystal, called plastic deformation, rather than from a trace element.

Human intervention can also add colour. Irradiation and HPHT (high pressure, high temperature) are among the treatments used to produce or change colour, and it describes lab-grown coloured diamonds made by HPHT or CVD. Treated and lab-grown stones can resemble natural fancy colours, which is why the origin of the colour matters as much as the shade.

A fancy colour is described by three things: hue (the colour itself), tone (how light or dark it looks) and saturation (how strong the colour is). The grade names are Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Deep and Fancy Dark, and not every colour uses every grade. GIA calls Fancy Vivid the most sought after for blue and the most prized for yellow, and notes that stones of medium tone usually show the strongest, liveliest colour.

The 4Cs still apply, but the priorities flip. For a white diamond, cut drives brilliance. For a fancy colour stone, GIA evaluates less for brightness and fire and more for the intensity and hue of the colour. A famous example is the Hope Diamond, a 45.52 ct stone that GIA graded Fancy Deep Grayish Blue.

Ask before anything else, because the three can look alike and are priced very differently. A GIA report designates a diamond's colour as natural or treated, and lab-grown coloured diamonds are made by HPHT or CVD, so ask for a report that states the origin of the colour.

Your protections depend on where you buy. In the United States, the FTC's Jewelry Guides say a laboratory-grown diamond should be described with a clear qualifier such as "laboratory-grown", and that a treatment should be disclosed when it is not permanent, needs special care or significantly affects value. Those are US guides, but a good seller anywhere should be able to say how a colour arose. A treated or lab-grown colour can be a perfectly reasonable choice when it is disclosed and priced as what it is. The risk is paying for natural rarity you did not receive.

One clue from GIA: treated yellow diamonds often have intensely saturated colour that looks a bit cooler or greener. It is a prompt to ask for the report, not proof.

Begin with the colour you love, then the budget, then the report. Yellow is the most common natural fancy colour, so it is easier to find; blue is far rarer, and Fancy Vivid grades are the most sought after. If you want the look without the rarity, treated and lab-grown stones exist, and a clearly labelled one is a fair choice.

Colour also depends on the setting. The metal, the surrounding stones and the cut change how the colour reads, so a ring is best judged in person. The custom engagement rings and custom jewellery design pages describe how a ring is planned around a chosen stone, and you can sketch the look in the design studio or talk it through in a showroom before you commit.