The Big Four

Carbon

Carbon crystallized in the cubic system. In 1953 GIA introduced its D-to-Z scale: D means no color at all, and towards Z a light yellow, light brown or light gray appears.

The yellowish tint comes mostly from nitrogen, the N3 defect, called «cape» after South Africa's Cape Province, the first great source of such diamonds. Brown comes from natural plastic deformation of the crystal lattice.

Corundum colored by chromium

Red corundum. Chromium causes its red, which ranges from an orangy red to a purplish red; the more chromium, the stronger the red. Chromium also gives it a red fluorescence that adds to the intensity of the color.

«Pigeon's blood» is not a single standard: since 2015 the SSEF and Gübelin laboratories reserve it for an intense, saturated and homogeneous red with no treatment at all; the Thai GIT accepts heating; and GIA advises avoiding the term because it can be misinterpreted. Almost all rubies are heat-treated.

Corundum colored by iron and titanium

Corundum that is not red, since red corundum is ruby. Its classic blue comes from a charge transfer between iron and titanium (Fe2+→Ti4+); the more iron, the darker the blue. Heating is common.

«Royal blue» is, for SSEF and Gübelin, a saturated blue, pure or with a very slight purplish tint, and untreated. It was coined for the sapphires of Burma and is now also used for other sources. A place name such as Kashmir tells the origin, never the color.

Beryl colored by chromium and/or vanadium

For the laboratories of the LMHC, emerald is a beryl colored mainly by chromium and/or vanadium, with a medium to strong green saturation. An iron-rich beryl whose green comes only from traces of chromium is a green beryl, not an emerald.

Chromium colors both ruby and emerald: in emerald, its surroundings in the crystal open a transmission window in the green, where the eye is most sensitive.