Green emerald. The mineral is transparent emerald, the green variety of beryl on calcite matrix. 2.5 x 2.5 cm. Coscuez, Boyacá, Colombia. Emeralds are the green, gem variety of the mineral beryl (Beryllium aluminum silicate). They are named after the Greek "smaragdos" which means "a light green precious stone," and are generally regarded as among the most precious of gems.

The term diagram of Cr³⁺ in a distorted octahedral field (A), the energy levels and transitions in ruby (B), and the resulting absorption spectrum and fluorescence of ruby (C) and emerald (D). In an emerald, the symmetry is distorted octahedral, but the ligand field is a little weaker: 2.05 eV compared to the 2.23 eV of ruby. This relatively small change produces a significant shift in the absorption bands, as shown at D, thus resulting in a change from the red color of ruby to the green color of emerald. Here, the violet and yellow-red regions of light passing through emerald are absorbed, as in the two upward arrows leading to the 4T1 and 4T2 levels, producing the absorption spectrum shown at D. Green-blue is transmitted to give the color of emerald. Again, the selection rules do not permit a return from the two excited states (4T1 and 4T2) directly to the ground state. In losing its energy again, the Cr³⁺ must pass through the state labeled 2E with the emission of some heat. In returning from 2E back down to the ground state, a quantum of red light, a red fluorescence similar to that of ruby, is emitted. Interestingly, the position of the 2E level involved in the fluorescence does not change significantly with the ligand field, as can be seen at A, so that the same red fluorescence is present in both red ruby (strongly) and green emerald (weakly).

Red fluorescence. White light (left) and ultraviolet (right) views of a mineral specimen from Franklin, New Jersey, containing calcite CaCO₃ (tan color with red fluorescence) and willemite ZnSiO₄ (brown with green fluorescence), both containing Mn²⁺, as well as the green synthetic emerald Be₃Si₆O1₈:Cr and the red synthetic ruby Al₂O₃:Cr, both showing the red Cr³⁺ fluorescence; the ruby is 2 cm across.

Yellow beryl (Heliodor) is colored by the presence of F³⁺ ions. Beryl includes emerald, aquamarine, and lesser-known varieties like goshenite (colorless), morganite (pink), Heliodor (yellow), and bixbite (red).

A synthetic alexandrite gemstone, 5 mm across, changing from a reddish color in the light from an incandescent lamp to a greenish color in the light from a fluorescent tube lamp.

The purple-orange dichroism (Cr³⁺ ligand field colors) in a 3-cm-diameter synthetic ruby; the arrows indicate the electric vectors of the polarizers. One result of the symmetry of the ligand field is "pleochroism," the existence of up to three colors in different directions in a crystal. The two "dichroic" colors, orange and purple, are seen with polarized light in a ruby.

The variation of the ligand field and the color in the mixed system of colorless sapphire Al₂O₃ and chrome green Cr₂O₃. As the chromium concentration of a ruby is increased, there is a change from red through gray to green as the ligand field becomes weaker; pure chromium oxide Cr₂O₃ is the dark-green pigment chrome green. A specimen on the red side of gray can be turned green by heating it to cause a reduction of the ligand field, as the atoms move apart from thermal expansion in "thermochromism"; in the reverse phenomenon "piezochromism," green can be turned red by the application of pressure. Not to scale.