Precious Metals

A precious metal is a rare metallic chemical element of high economic value. Chemically, the precious metals are less reactive than most elements, have high luster, are softer or more ductile, and have higher melting points than other metals. Historically, precious metals were important as currency, but are now regarded mainly as investment and industrial commodities. Gold, silver, platinum and palladium each have an ISO 4217 currency code.

The best-known precious metals are gold and silver. While both have industrial uses, they are better known for their uses in art, jewellery and coinage. Other precious metals include the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium and platinum, of which platinum is the most widely traded. Rhenium is a precious metal that is not part of the platinum group or one of the traditional precious metals.

The demand for precious metals is driven not only by their practical use, but also by their role as investments and a store of value. Palladium, as of January 29, 2008, is valued at $388 USD per troy ounce, which is less than half the price of gold, at $995 USD/ounce. Platinum's price of $2,032 USD/ounce is more than twice that of gold. Rhodium is usually the most expensive of the precious metals, over 4.5 times more expensive than platinum, with a current price of $7,150. However, the price of rhenium has been fluctuating wildly recently and at the moment it is selling at 342 USD per ounce ($11,000 per kilogram, 32.15 Troy ounces per kilogram, July 2008). Meanwhile, ruthenium is valued at $525 USD per ounce and silver has fluctuated as well lately, trading at $16.69 USD/ounce. Silver is substantially less expensive than all of the other precious metals, presently less than 1/50 the price of gold, but is often traditionally considered a precious metal for its role in coinage and jewellery.

Fools Gold

Black gold or 'grey gold' as it is sometimes referred to is a type of gold used in jewelry . Though gold is known for its warm yellow color it can be produced by various methods in differing colors ranging from white to black . Black colored gold can be produced by various methods.

• Electrodeposition or Electroplating using black rhodium or ruthenium. Solutions that contain ruthenium give a slightly harder black coating than those that contain rhodium.
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• Patination by applying sulfur and oxygen containing compounds.
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• Plasma assisted chemical vapour deposition process involving amorphous carbon, and controlled oxidation of carat gold containing chromium or cobalt
  More recently a laser technique has been developed that renders the surface of metals deep black. A so called femtosecond laser pulse deformes the surface of the metal forming nanostructures. The immensely increased surface area can absorb virtually all the light that falls on it and thus rendering it deep black.

Mineralogy

This mineral occurs as isometric crystals that usually appear as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a result of alternation of the cube and pyritohedron faces. Pyrite also frequently occurs as octahedral crystals and as pyritohedra (a dodecahedron with pentagonal faces). It has a slightly uneven and conchoidal fracture, a hardness of 6–6.5, and a specific gravity of 4.95–5.10.It is brittle and can be identified in the field by the distinctive odor released when samples are pulverized.

Pyrite is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as a replacement mineral in fossils. Despite being nicknamed fool's gold, small quantities of gold are sometimes found associated with pyrite. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin, Nevada gold deposit, arsenian pyrite contains up to 0.37 wt% gold.Auriferous pyrite is a valuable ore of gold.

Weathering and release of sulfate

Pyrite exposed to the atmosphere during mining and excavation reacts with oxygen and water to form sulfate, resulting in acid mine drainage. This acidity results from the action of Acidithiobacillus bacteria, which generate their energy by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) using oxygen. The ferric iron in turn attacks the pyrite to produce ferrous iron and sulfate. The ferrous iron is then available for oxidation by the bacterium; this cycle continues until the pyrite is depleted.

Uses

Pyrite is used commercially for the production of sulfur dioxide, for use in such applications as the paper industry, and in the manufacture of sulfuric acid, although such applications are declining in importance.

Pyrite and marcasite

Pyrite is often confused with the mineral marcasite, a mineral whose name is derived from the Arabic word for pyrite, due to their similar characteristics. Marcasite is a polymorph of pyrite, which means it has the same formula as pyrite but a different structure and, therefore, different symmetry and crystal shapes. The formal oxidation states are, however, the same as in pyrite because again the sulfur atoms occur in persulfide-like pairs.Marcasite/pyrite is probably the most famous polymorph pair next to the diamond graphite pair. Appearance is slightly more silver.

Marcasite is metastable relative to pyrite and will slowly be changed to pyrite if heated or given enough time. Marcasite is relatively rare, but may be locally abundant in some types of ore deposits, such as Mississippi Valley-type Pb-Zn deposits. Marcasite appears to form only from aqueous solutions.

Pyrite is often used in jewelry such as necklaces and bracelets, but although the two are similar, marcasite cannot be used in jewelry as it tends to crumble into powder. Adding to the confusion between marcasite and pyrite is the use of the word Marcasite as a jewelry trade name. The term is applied to small polished and faceted stones that are inlaid in sterling silver, but even though they are called marcasite, they actually contain pyrite.

Formal oxidation states for pyrite, marcasite, and arsenopyrite

From the perspective of classical inorganic chemistry, which assigns formal oxidation states to each atom, pyrite is probably best described as Fe²⁺S^₂2-. This formalism recognizes that the sulfur atoms in pyrite occur in pairs with clear S-S bonds. These persulfide units can be viewed as derived from hydrogen persulfide, H₂S₂. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. In contrast, molybdenite, MoS₂, features isolated sulfide (S^2-) centers. Consequently, the oxidation state of molybdenum is Mo⁴⁺. The mineral arsenopyrite has the formula Fe As S. Whereas pyrite has S₂ subunits, arsenopyrite has AsS units, formally derived from deprotonation of H₂AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe³⁺AsS^3-.
 

The Jewels of Jacqueline Kennedy Onassis by Ellen Steiber Part 5

The Jewels of Jacqueline Kennedy Onassis by Ellen Steiber Part 4

The Fashion Jewelry

Not everyone realizes that while Jacqueline Kennedy Onassis owned and wore some of the finest gems in the world, she was also a great fan of costume jewelry. Many of the classic photographs of her show her wearing what were then relatively inexpensive costume pieces. It’s The Fashion Jewelry section of the catalog that features many of the strands of faux pearls that she made so famous.

The first “fashion jewel” in the catalog is the triple strand of faux pearls with an Art Deco style clasp that Mrs. Kennedy wore in the White House. The photograph of the necklace is accompanied by a 1962 shot of Jackie holding her then two-year-old son. He’s playing with a strand of the necklace as she tilts her head back, laughing. It’s a tender photograph, an almost idyllic glimpse of motherhood. This famous necklace was estimated at $500-$700, a low-ball price even if the pearls were made of paste. It sold for $211,500.

Though Jackie wore costume jewelry, a good deal of it was designer costume jewelry. There’s a signed Valentino brooch in the shape of a large crab with a faux topaz body and faux pavé diamond legs; a gilt metal cross by Christian Lacroix; simple “silvered metal balls” earclips by Yves Saint Laurent; and faux emerald and diamond earclips by Chanel.

One of the things that becomes clear from the catalog photographs was that Jackie had no qualms about wearing costume jewelry for even the most important occasions. There’s a photo of her speaking with France’s President Charles de Gaulle, during the Kennedys’ 1961 trip to Paris. Mrs. Kennedy is wearing a simple white dress and what Sotheby’s describes as a “black ‘stone’ bead double strand necklace.” She also wore this necklace on the day her husband announced his candidacy and while they were in the White House. It wasn’t a fancy piece, and I’m guessing the beads were made of glass, but somehow it accented her white dress perfectly. Sold with a single black earclip (the other earring must have gone missing), the set was estimated as being worth $200-$300. A true piece of history, it sold for $101,500.

The catalog also offers two pairs of earrings, shaped to look like miniature conch shells. One pair, little pearlescent conches with gilt accents, was designed by Kenneth Jay Lane, who was known for his fabulous fakes. Jackie is shown wearing them in 1967 as she and Caroline gazed at the naval carrier, The U.S.S. John F. Kennedy, which Caroline had just christened. This pair of earclips, along with a second pair in a similar design, were estimated at $400-$600 and sold for $25,300. Other pieces were not quite so historic and yet carried Jackie’s charm. An ebony and ivory beaded necklace along with two ivory cuff bracelets and a pair of ivory cabochon earrings were estimated at $500-$700 and sold for $14,950. A choker made of four twisted strands of gold pearls, black pearls, and round crystal beads—all simulated— along with a pair of faux pearl earclips, was estimated at $150-$250 and sold for $17,250. Another necklace, a single strand of fake hematite beads with gilt roundels had an estimated worth of $150-$250 and sold for $13,800. Two strands of melon-shaped green glass beads—on Jackie, they probably were assumed to be emeralds--along with a pair of earrings, were valued at $300-$400 and sold for $20,700.