19-12-2012, 01:36 PM
SPINEL DEPOSITS
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Spinel (MgAl2O4) is a much maligned gem mineral that is often thought of as a corundum simulant because in red and blue varieties it can closely resemble ruby or sapphire. It is, however, an outstanding gem material in its own right; it is hard (8), possesses an intermediate to high R.I., shows good dispersion, and is available in a wide variety of colors. Many of the worlds most famous large "rubies" are, in fact, red spinel (e.g. Black Princes' Ruby and Timur Ruby, both in British Crown Jewels). Such stones were once referred to as Balas Ruby, a term that is not used today but is synonymous with red spinel. The confusion is well-founded; the finest red spinels come from the same gem gravel deposits in Myanmar that are renowned for "Burma" ruby, and both share somewhat similar optical and physical properties.
The Black Prince’s Ruby now is mounted in the front of the Imperial State Crown, just above the famous Cullinan II Diamond. It is a huge, semi-polished octahedron.* Sitwell15 states that the stone is backed by a gold foil, as were many ancient gems, to improve its brilliance. This has not been removed for fear of damaging the gem. The stone measures some two inches (5.08 cm) in length and is of proportionate width.14 Its exact weight is unknown, but estimates put it at ~140 ct.
Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.
[attachment=44358]
Spinel (MgAl2O4) is a much maligned gem mineral that is often thought of as a corundum simulant because in red and blue varieties it can closely resemble ruby or sapphire. It is, however, an outstanding gem material in its own right; it is hard (8), possesses an intermediate to high R.I., shows good dispersion, and is available in a wide variety of colors. Many of the worlds most famous large "rubies" are, in fact, red spinel (e.g. Black Princes' Ruby and Timur Ruby, both in British Crown Jewels). Such stones were once referred to as Balas Ruby, a term that is not used today but is synonymous with red spinel. The confusion is well-founded; the finest red spinels come from the same gem gravel deposits in Myanmar that are renowned for "Burma" ruby, and both share somewhat similar optical and physical properties.
The Black Prince’s Ruby now is mounted in the front of the Imperial State Crown, just above the famous Cullinan II Diamond. It is a huge, semi-polished octahedron.* Sitwell15 states that the stone is backed by a gold foil, as were many ancient gems, to improve its brilliance. This has not been removed for fear of damaging the gem. The stone measures some two inches (5.08 cm) in length and is of proportionate width.14 Its exact weight is unknown, but estimates put it at ~140 ct.
Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.