EMERALD General; Category Beryl variety Formula (repeating unit) Be3Al2(SiO3)6 Crystal symmetry (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 Crystal system Hexagonal (6/m 2/m 2/m) Space group: P6/mсc 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 emeralds 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. Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of Connoisseurship: Color, Cut, Clarity 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. Both are necessary conditions. 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. Color In gemology, color is divided into three components: hue, saturation and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue necessarily being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emerald; light-toned gems are known instead by the species name green beryl. The finest emerald are approximately 75% tone on a scale where 0% tone would be colorless and 100% would be opaque black. In addition, a fine stone should be well saturated; the hue of an emerald should be bright (vivid). Gray is the normal saturation modifier or mask found in emerald; a grayish-green hue is a dull green hue. Emeralds are green by definition (the name is derived from the Greek word smaragdus, meaning green). Emeralds are the green variety of beryl, a mineral which comes in many other colors that are sometimes also used as gems, such as blue aquamarine, yellow heliodor, pink morganite, red beryl or bixbite, not to be confused with bixbyite, and colorless goshenite. Clarity Emerald tends to have numerous inclusions and surface breaking fissures. Unlike diamond, where the loupe standard, i.e. 10× magnification, is used to grade clarity, emerald is graded by eye. Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated (oiled, see below) to enhance the apparent clarity. The inclusions and fissures within an emerald are sometime described as the garden, because of their mossy appearance. These imperfections within the stone are unique to each emerald and can be used to identify a particular stone. Eye-clean stones of a vivid primary green hue (as described above) with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone command the highest prices. This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes. Faceted Emeralds are most commonly given the Oval cut, or the signature Emerald cut, a rectangular cut with facets around the top edge.
Posted on: Mon, 24 Nov 2014 12:15:27 +0000
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