Blue (hue / variety / modifier): Abbreviated (B). A perceived colour hue resembling the sky on a clear day or the gemstone lapis lazuli. Blue is a primary colour of both pigment and light. It is one of the spectrum colours, positioned between violet and green, in the 430 nm to 490 nm wavelength region. Blue is often called a cold colour. In the diamond trade, blue represents one of the basic twelve colour varieties. Blue diamonds can also contain secondary colour modifiers such as blackish blue (bk-B), grayish blue (gy-B), and greenish blue (g-B). Blue can also occur as a secondary colour modifier in other colour varieties such as bluish green (b-G), bluish gray (b-GY), etc. The range of colour tone for blue diamonds is variable, especially in lightness, extending from very light (powder) blue through light (sky) blue, medium (bluebird) blue, dark (navy) blue, to very dark (ink) blue. Using the Rennilson-Hale Gemstone Colorimeter, the author found that the majority of natural blue (type llb) diamonds are in the weak (Wk) saturation range and thus exhibit a tinge of gray in their inherent body colour (Hofer 1989 & 1991). Saturation in these blue diamonds is limited to weak (L*=00 – 10) grayish “steel” blue (gy-B) and weak-moderate (L*=10 – 20) pure “sapphire” blue (B) colours. Other blue diamonds are coloured naturally by irradiation (Koivula 1993wn). These nonconductive (type la or Ila) blue diamonds usually exhibit a greenish colour modifier, often referred to as “teal” blue colour (see Sotheby’s NY Apr 1994 Lot 433). Other nonconductive blue diamonds exhibit gray colour modifiers resulting from hydrogen impurities (Fritsch 1992). Popular names for different mixtures of blue are: ajer-laut, aquamarine, azure, baby, blueberry, bluebird, cornflower, hydrangea, indigo, ink, Italian ice, midnight, navy, powder, Prussian, robin ‘s-egg, royal, sapphire, sea, sky, steel, teal, turquoise, Wedgwood, and zircon

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