The color of gemstones is one of their most characteristic features; and, equally, constitutes one of the most important, for it is color which strikes human eyes most strongly. It is produced in gemstones by the absorption of light. Visible white light, as is known, is a mixture of the spectral or rainbow colors from red through orange, yellow, yellow-green, green, blue-green, to blue, over a spread of wavelengths from 800 nm (= red) to 400 nm (= violet). One nm (nanometer) = one millionth of a millimeter.
The individual color components of which white light is composed are variously absorbed in gemstones according to the atomic structures proper to the gemstone varieties and their chemical composition. This absorption of light is brought about by trace elements - also called transition elements, of which the most important and the commonest are iron, chromium, copper, nickel and vanadium. As a consequence of a partial removal of wavelengths (= selective absorption) from the total spesurn a mixed color emerges which results from the non absorbed wavelengths (= colors). Thus, for example, a gemstone looks red when only those wavelengths whose residual mixture produce a red color are transmitted and strike the eye of the observer. On the other hand a gemstone appears colorless when almost all the wavelengths of white light 'are more or less equally strongly absorbed.
The color palette of diamond is very richly varied; it ranges from pink through red, blue, green, yellow and brown even to black diamonds. The rarest color occurring in natural diamonds is red, followed by green, blue, purple and brown.
The color of a diamond depends on the quantity and nature of the included trace elements and how these originated. Most gemstones occur in color hues ranging from colorless through pale yellowish, greenish yellow to brownish yellow. The color of these diamonds is caused by the presence of nitrogen atoms. Diamonds containing nitrogen are classified structurally in the group of Type I a diamonds. Less common are diamonds of Type I b which have isolated nitrogen atoms. These diamonds are to be found in stones of yellow, brown and orange colors.
The blue color is caused by the presence of atoms of the element boron. They are especially rare and are classified under Type II. Their electrical conductivity is a characteristic feature.
The color in naturally green diamonds probably comes from the natural presence of radioactive material like uranium in the immediate vicinity of the location where the diamond originated.
The very rare natural color of pink or pale pink and also pink tints blending into brownish ones are presumably caused by high temperatures or the effects of pressure during growth and this gives rise to tensions and deformations in the structure of the crystal lattice.
Until recently pink-colored diamonds with pale-pink tints came from the deposits in Brazil, India and Africa. The new deposits of the Argyle Mine in Australia caused surprise, with their deep pink tints which can range to burgundy red. They are cut directly on the site by specially trained cutters in very fancy shapes which show their unusual color to best advantage. Pink and reddish diamonds from the Argyle Mine have likewise fetched fancy prices at international auctions in recent years.
Colored diamonds in the trade go under the name of "fancy colors". Because of their rarity they are held in very high esteem, especially when the color displays an intense saturation. The most valuable diamonds, however, are the completely colorless stones - in this respect diamond is the only gemstone whose colorlessness renders it more valuable.
Contrasting with the intensely colored diamonds are the far more common naturally occurring diamonds with a slight to obvious yellow saturation. In commercial color grading these yellow tinted diamonds are grouped in the first place with the colorless stones. This group constitutes the so-called "cape".
Diamond Color Chart
- Color Grades - Origin and Nomenclatures
- Comparison Brilliants as the Basis of Color-Grading
- Introduction to Practical Color-Grading
- Instruments for Color-Grading
- Artificial Color changes in diamonds and their methods of identification