Color in the stone

GIA describes color in terms of hue, the color itself; tone, its relative lightness or darkness; and saturation, its intensity. The same hue can look very different: a sapphire can be a light, a medium or a very dark blue, and more or less vivid. That is why every gem on Pigmentoria is shown in four tones: the hue it is named by, its light part, its deepest part and the sparkle where light bounces off the facets.^[1]

In 1953 GIA introduced its D-to-Z scale for diamonds. D is colorless; towards Z a light yellow, light brown or light gray appears. GIA begins at D to avoid confusion with earlier systems, and grades each stone face down, under controlled light, against master stones.

The scale has five groups: colorless (D–F), near-colorless (G–J), faint (K–M), very light (N–R) and light (S–Z). Beyond that, a diamond with enough color is graded as a fancy color.^{[2, 3]}

A colored diamond is graded by the presence of color, not its absence. GIA chooses one of 27 hues and adds a term that sums up the combined effect of tone and saturation: Fancy Light, Fancy, Fancy Intense, Fancy Deep, Fancy Vivid or Fancy Dark. So a pink can be Fancy Light or Fancy Vivid, and the second is far more intense.^{[1, 4]}

Most gem colors come from small amounts of a metal. Chromium makes ruby red and emerald green: in emerald, its surroundings in the crystal open a window of light in the green. A charge transfer between iron and titanium makes sapphire blue; manganese makes morganite pink and red beryl red; copper colors turquoise.

Other colors come from defects. Natural radiation creates the color of smoky quartz and amethyst. And some come from light itself: the play of color of opal is light diffracted by ordered silica spheres, the orient of pearl is interference in its layers, and the glow of moonstone is light scattered by very thin layers of feldspar.^{[5, 6, 7]}

Some trade names have a defined meaning. For the laboratories of the LMHC, padparadscha is a pinkish orange to orangey pink corundum of any origin, which must pass a color-stability test, and Paraíba tourmaline is a tourmaline colored mainly by copper and manganese, also of any origin.

«Pigeon’s blood» and «royal blue» are defined by individual laboratories: SSEF and Gübelin keep them, since 2015, for intense, saturated, homogeneous and untreated rubies and sapphires, compared against master stones. GIA advises avoiding «pigeon’s blood» because it can be misinterpreted. A place name such as Kashmir or Burma tells the origin, as an opinion, never a color or a level of quality.^{[8, 9, 10, 11, 12]}

Color depends on the light. The LMHC judges padparadscha in standard daylight, between D50 and D65, and defines a color change by comparing daylight, from 5500 to 6500 K, with incandescent light, from 2700 to 3600 K: alexandrite is greenish in the first and reddish or purplish in the second.

Many colors on the market are the result of treatment. Blue topaz is almost always treated; most citrine is heated amethyst; and, according to CIBJO, tanzanite is almost always heated and blue zircon always is. That is why each entry on Pigmentoria says when a color is usually treated.^{[8, 13, 14, 15]}

Color gives clues about what causes it, but it is not enough to know where a stone comes from, whether it has been treated or whether it is natural: that takes a laboratory, and even then its findings are opinions, as the LMHC states in its reports. The colors on Pigmentoria are our own digital approximations: a guide for looking, not a laboratory standard.^[16]