Emeralds

Emerald
Emerald crystal from Muzo, Colombia
General
Category	Beryl variety
Formula (repeating unit)	Be3Al2(SiO3)6
Crystal system	Hexagonal (6/m 2/m 2/m) Space group: P6/mсc
Space group	(6/m 2/m 2/m) – Dihexagonal Dipyramidal
Unit cell	a = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Formula mass	537.50
Color	Green shades to colorless
Crystal habit	Massive to well Crystalline
Cleavage	Imperfect on the [0001]
Fracture	Conchoidal
Mohs scale hardness	7.5–8
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	Average 2.76
Optical properties	Uniaxial (-)
Refractive index	nω = 1.564–1.595, nε = 1.568–1.602
Birefringence	δ = 0.0040–0.0070
Ultraviolet fluorescence	None (some fracture filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References

Emerald is a gemstone and a variety of the mineral beryl (Be₃Al₂(SiO₃)₆) colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.

The word "emerald" is derived (via Old French: esmeraude and Middle English: emeraude), from Vulgar Latin: esmaralda/esmaraldus, a variant of Latin smaragdus, which originated in Ancient Greek: σμάραγδος (smaragdos; "green gem").

Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of Connoisseurship: Color, Clarity, Cut and Carat weight. Before the 20th century, jewelers used the term water, as in "a gem of the finest water", to express the combination of two qualities: color and clarity. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, clarity is considered a close second. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.

In the 1960s, the American jewelry industry changed the definition of "emerald" to include the green vanadium-bearing beryl as emerald. As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe. In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as "Colombian Emerald".