Trapiche emerald is a rare variety of the gemstone

emerald Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.Hurlbut, Cornelius S. Jr., and Kammerling, Robert C. (1991). ''Gemology'', John Wiley & Sons, New York ...

, characterized by a six-arm radial pattern of usually black spokes separating areas of green emerald. If weathered, the black spokes may become light in color. Trapiche emerald is one of an assortment of trapiche or trapiche-type minerals. Others include trapiche

ruby Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

tourmaline Tourmaline ( ) is a crystalline silicate mineral, silicate mineral group in which boron is chemical compound, compounded with chemical element, elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, and chiastolite. The name comes from the Spanish term '' trapiche'', a sugar mill, because of the resemblance of the pattern to the spokes of a grinding wheel. Emerald is a gem variety of the mineral

beryl Beryl ( ) is a mineral composed of beryllium aluminium Silicate minerals#Cyclosilicates, silicate with the chemical formula Be3Al2(SiO3)6. Well-known varieties of beryl include emerald and Aquamarine (gem), aquamarine. Naturally occurring Hex ...

, and owes it distinctive green color to the presence of

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

and/or

vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

. Trapiche emeralds were first described by Émile Bertrand in 1879. With few exceptions, they are found in the western part of the Eastern Cordillera basin, in the

Muzo Muzo () is a town and municipality in the Western Boyacá Province, part of the department of Boyacá, Colombia. It is widely known as the world capital of emeralds for the mines containing the world's highest quality gems of this type. Muzo ...

, Coscuez, and Peñas Blancas mines of Colombia, where they are rare. Although reported in older literature from Chivor, Colombia, that mining district is now thought to be an unlikely source. Extremely rare finds in Brazil and Madagascar have been reported. The radial pattern in any individual trapiche emerald crystal exhibits a moderate amount variance, depending in part on distance from the termination of the crystal, and on where the cross-sectional slice of the crystal is taken. Some slices through trapiche emeralds have a hexagonal structure at the core. There is not yet consensus about the mechanism by which the radial pattern forms, or the conditions required for it. Multiple models have been proposed. According to one interpretation the black radial arms are developed from clay minerals in the matrix where the emeralds formed. The clay matrix would later lithify to form

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

. During growth of a "Type A" trapiche emerald, the clay minerals formed radial dendrites on which layers of emerald would grow. A "Type B" trapiche emerald exhibits the reverse growth pattern. A trapiche pattern is a fixed-star pattern in a mineral, and differs from asterism, which is a moving-star pattern in a mineral. Asterism is the result of tiny mineral inclusions, minute tubes, or nanovoids, oriented with the crystallographic directions of the mineral. In the case of corundum (ruby and sapphire), when light strikes the inclusions or nanovoids, it is both reflected and refracted, producing a multidirectional chatoyance.

References

{{reflist Beryl group

See also

References