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The carat ( ct) is a unit of mass equal to 200 mg (0.00705 oz; 0.00643 ozt ), which is used for measuring gemstones and pearls . The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures, [ 1][ 2] and soon afterwards in many countries around the world.
The stone or stone weight (abbreviation: st.) [1] is an English and British imperial unit of mass equal to 14 avoirdupois pounds (6.35 kg). [nb 1] The stone continues in customary use in the United Kingdom and Ireland for body weight . England and other Germanic -speaking countries of Northern Europe formerly used various standardised "stones ...
Opal is a hydrated amorphous form of silica (SiO 2 · n H 2 O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur ...
Struvite. Struvite ( magnesium ammonium phosphate) is a phosphate mineral with formula: NH 4 MgPO 4 ·6H 2 O. Struvite crystallizes in the orthorhombic system as white to yellowish or brownish-white pyramidal crystals or in platy mica -like forms. It is a soft mineral with Mohs hardness of 1.5 to 2 and has a low specific gravity of 1.7. It is ...
Gemstones of the World revised 5th edition, 2013 by Walter Schumann ISBN 978-1454909538 Smithsonian Handbook: Gemstones by Cally Hall, 2nd ed. 2002 ISBN 978-0789489852 v
A collection of gemstone pebbles made by tumbling the rough stones, except the ruby and tourmaline, with abrasive grit inside a rotating barrel. The largest pebble here is 40 mm (1.6 in) long. The traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious; similar ...
Pyrite cubic crystals on marl from Navajún, La Rioja, Spain (size: 95 by 78 millimetres [3.7 by 3.1 in], 512 grams [18.1 oz]; main crystal: 31 millimetres [1.2 in] on edge) Pyrite's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold.
The aquamarine crystals are then sorted according to size, shape, color, and clarity following the initial processing. The gemstones are assessed and graded by gemologists and experts according to predetermined standards, such as the four C's (color, clarity, cut, and carat weight).
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