Yellow diamonds owe their colour to nitrogen. Since nitrogen increases surface tension in diamonds, yellow diamonds are usually well crystallized. They also tend to be softer on the polishing scaife.
Different arrangements of Nitrogen atoms throughout the crystal lattice cause various intensities of yellow seen in diamonds. Yellow is often the result of a selective absorption pattern featuring ‘cape’ lines, named after the Cape Province of South Africa which hosts deposits rich in light yellow diamonds. These cape lines show up in the blue part of the visible light spectrum at 478Nm, caused by N2 and at 415Nm due to N3 nitrogen aggregates. These are groups of two (Type IaA) or three nitrogen atoms or 4 Nitrogen atoms around a vacancy (Type IaB). They recombine the returning light wavelengths as ‘yellow’ to the viewer.
Cape yellow diamonds represent 99% of all fancy yellow diamonds and despite a high content of Nitrogen of up to 3000Ppm (parts per million) only rarely reach intense colours and mostly in larger sizes. Nonetheless ‘cape yellows’ remain quite attractive since yellow in diamonds reaches its optimum at what would otherwise be considered a relatively low level of colour saturation.
The majority of intense, vivid and deep yellow stones, however, belong to the remaining 1%. These Type Ib diamonds typically contain less than 100ppm of isolated nitrogen atoms producing a characteristic absorption pattern in the violet light spectrum. Near to complete absorption of light below 480Nm gives rise to an extraordinary rich yellow colour to which all other yellow gems of this world compare.
Fluorescence in yellow diamonds is common. Nil or faint fluorescence are preferred as a medium to strong blue ‘fluo’ may weaken the face-up colour. GIA sometimes penalizes such stones by assigning them a lower than possibly expected intensity grade. Yellow fluorescence may, however, enhance the body colour and is desirable as long as it does not impede the crystal. Old Golconda Yellows are an excellent example. Same is true to a lesser extent for orange fluorescence which may intensify the hue whilst shifting it.
Uneven colour zoning is often seen in yellow diamonds.
The Gemological Institute of America recognizes fancy light, fancy, fancy intense, fancy vivid, fancy deep and fancy dark yellow saturation levels. Observations lead to the conclusion that the GIA has recently considerably tightened colour grading standards for yellow diamonds.
The global market generally prefers pure as well as orangy to orange yellows which may be traded at a significant premium. Green is another modifier seen and depending on true appearance may either be desirable or turn the diamond drab. Although all secondary colours reduce the apparent saturation of yellow stones, none are however less desirable than either brown or worse unattractive grey (hydrogen). Brownish yellow diamonds are still generally well received due to their warm golden colour.
The most valuable yellow varieties in absolute terms – all other factors being equal – are phosphorescent neon green to greenish yellow lime coloured stones. Truly exceptional specimens may command premiums of easily between 100 to 200% above prices quoted for highly desired vivid to deeply saturated orange-yellow, orangy as well as pure Yellow stones. The best value-for-money stones are well saturated fancy yellows and intense yellows.
The Zimmi area of Sierra Leone and bordering Manu river in Liberia is particularly famous for supersaturated canary or jonquille (daffodil hued) yellows. However beautiful intense and vivid yellow diamonds are also found at Ellendale in Western Australia, in Northern Canada, the Grib mine of Russia, in Indonesia (Borneo), Lesotho and Botswana.
Large ‘ordinary’ cape Yellows are commonly recovered in South Africa, Angola and at the Yakutian mines in Russia.
Due to their relative abundance, yellow diamonds are more severely graded in terms of clarity relative to other fancy colours. In terms of pricing, dealers broadly differentiate between three categories: SI, VS and up, as well as IF which may command a special premium.
Yellow diamonds are commonly set in jewellery backed by 22 to 24ct yellow gold which strongly intensifies the visual appearance.