Chrysoprase

Chrysoprase (also chrysophrase) is a gemstone variety of chalcedony (fibrous form of quartz) that contains small quantities of nickel. Its color is normally apple-green, but varies to deep green. It is cryptocrystalline, which means that it is composed of crystals so fine that they cannot be seen as distinct particles under normal magnification. This sets it apart from rock crystal, amethyst, citrine, and the other varieties of crystalline quartz which are basically transparent and formed from easily recognized six-sided crystals. Other members of the cryptocrystalline quartz family include agate, carnelian, and onyx. Unlike many non-transparent members of the quartz family, it is the color of chrysoprase, rather than any pattern of markings, that makes it desirable. The word chrysoprase comes from the Greekchrysos meaning 'gold' and prason, meaning 'leek'.

 Due to its comparative scarcity and pleasing green color, chrysoprase is one of the most prized varieties of quartz. Higher quality specimens often rival fine jade, for which it is sometimes mistaken. Cut into cabochons (smooth domed gems with flat backs for use in jewelry), it can be as sought after as fine amethyst.
 Unlike emerald which owes its beautiful green color to the presence of chromium, the color of chrysoprase is due to trace amounts of nickel in the structure. The nickel reportedly occurs as platelets of the talc-like mineral willemseite. Chrysoprase results from the deep weathering or lateritization of nickeliferous serpentinites or other ultramaficophiolite rocks. In the Australian deposits, chrysoprase occurs as veins and nodules with brown goethite and other iron oxides in the magnesite-rich saprolite below an iron and silica cap.

As with all forms of quartz, chrysoprase has a hardness of 6 - 7 on the Mohs hardness scale and a conchoidal fracture like flint. The best known sources of chrysoprase are Queensland, Western Australia, Germany, Poland, Russia, Arizona, California, and Brazil.

 

Emerald

Emerald (Be 3Al 2(SiO 3) 6) is a variety of the mineralberyl, colored green by trace amounts of chromium and sometimes iron. It is highly prized as a gemstone and by weight is the most valuable gemstone in the world, although it is often made less so by inclusions, which all emeralds have to some degree. Beryl has a hardness of 7.5 on the 10 point Mohs scale of hardness. However, this Mohs rating can decrease, depending on the number and severity of inclusions in a particular stone.

Most emeralds are oiled as part of the post lapidary process. The amount of oil entering an emerald microfissure is roughly equivalent to the size of a period in print.

Emeralds come in many shades of green and bluish green. There is a wide spectrum of clarity, along with various numbers of inclusions. Most emeralds are highly included, so it is quite rare to find an emerald with only minor inclusions. Because of the usual inclusions, the toughness (resistance to breakage) is classified as generally poor.

Emeralds in antiquity were mined by the Egyptians and in Austria as well as Swat in northern Pakistan.

A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. It is named for the trapiche, a grinding wheel used to process sugarcane in the region.

Cultural and historical/mythical usage
Emerald is regarded as the traditional birthstone for May , as well as the traditional gemstone for the astrological signs of Taurus and Cancer.
In some cultures, the emerald is the traditional gift for the 55th wedding anniversary. It is also used as a 20th and 35th wedding anniversary stone.

Famous Emerald
Gachala Emerald (origin: Colombia)
(The Gachala Emerald is one of the largest gem emeralds in the world at 858 carats. This stone was found in 1967 at La Vega de San Juan mine in Gachalá, Colombia. It is housed at the Smithsonian Institution in Washington DC.)
Chalk Emerald (origin: Colombia)
Duke of Devonshire Emerald (origin: Colombia)
Mackay Emeral

Emerald is a rare and valuable gemstone and, as such, it has provided the incentive for developing synthetic emeralds. Both hydrothermal and flux-growth synthetics have been produced and a method has been developed for producing an emerald overgrowth on colorless beryl. The first commercially successful emerald synthesis process was that of Carroll C. Chatham. Because Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium, a lithium vanadate flux process is probably involved. The other large producer of flux emeralds is Pierre Gilson Sr. which has been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds which become coated on both sides. Growth occurs at the rate of 1 mm per month and a typical seven-month growth run produces emerald crystals of 7 mm of thickness (Nassau, K. Gems Made By Man, 1980).

Hydrothermal synthetic emeralds have been attributed to IG-Farben, Nacken, Chatham and others but the first satisfactory commercial product was that of Johann Lechleitner of Inbruck, Austria, which appeared on the market on the 1960's. These stones were initially sold under the names "Emerita" and "Symeralds" and they were grown as a thin layer of emerald on top of natural colorless beryl stones. Although not much is known about the original process, it is assumed that Leichleitner emeralds were grown on acid conditions. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents (US3,567,642 and US3,567,643) acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating. Also, it is important that the silicon containing nutrient be kept away from the other ingredients in order to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. Typical growth conditions include pressures of 700-1400 bars at temperatures of 500 to 600 °C with a temperature gradient of 10 to 25 °C. Growth rates as fast as 1/3 mm per day can be attained.

Flux-grown synthetic emeralds fluoresce a dull red with long waveultraviolet light, due to an indicator added during the process of synthesizing the emerald, whereas natural specimens do not.

Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is exactly same as its natural counterparts. The Federal Trade Commission (FTC) has very strict regulations as to what can and what can not be called "synthetic" stone. The FTC says: "...[created stone must have] essentially the same optical, physical, and chemical properties as the stone named."[2] Furthermore, all natural emeralds, with the exception of the red Bixbite beryls from Utah which are anhydrous, have water inclusions, as emerald is of hydrothermal origin. Flux synthetic emeralds have no water, an integral part of any natural beryl (this also accounts for flux-grown emeralds being more stable when subjected to high temperatures). Hydrothermally-grown emeralds, however, contain water molecules. Wispy veil-like inclusions are common in flux-grown synthetic emeralds.

 

Feldspar

 Of all of the minerals in the Earth's crust, the aluminosilicate, feldspar, is the most common. It is a very useful industrial mineral that finds applications in making glass, plumbing fixtures, tile and pottery. These uses in the United States accounted for about 630,000 ton of U.S. produced feldspar valued at about $27.4 million. It is also a gemstone. Production of gem-quality feldspar in the United States increased rapidly in the past 5 years, declined slightly in 1993, but should continue to increase significantly in the future. Production was valued at more than $0.7 million in 1993. In the United States, gem feldspars are found in igneous, metamorphic, and sedimentary deposits from coast to coast and border to border. The feldspar group has nine species; there are varieties within some of the species, and there are also inter growth of species resulting from the un mixing of solid solutions. All of these species, varieties, and inter growths can be gemstones. Additionally, there are feldspars that have general names because of unique optical phenomena; these are sunstones and moonstones. Fine, gem-quality feldspar is very rare and some species and varieties are more common as gems than others.

Compositions 
This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
K-feldspar endmember KAlSi 3O 8
Albite endmember NaAlSi 3O 8
Anorthite endmember CaAl 2Si 2O 8
Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth.

Sanidine (monoclinic), orthoclase, and microcline (triclinic) refer to polymorphs of K-feldspar. Sanidine is stable at the highest temperatures, and microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites are coarse enough to be visible to the naked eye.

Compositions of the plagioclase series have been labeled as follows (percent anorthite in parentheses):

albite (0 to 10)
oligoclase (10 to 30)
andesine (30 to 50)
labradorite (50 to 70)
bytownite (70 to 90)
anorthite (90 to 100)

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae

Uses
Feldspar is a common raw material in the production of ceramics.
Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
Feldspar is an ingredient in Bon Ami brand household cleaner.
it is used as a glazing material

Oregon 
The vivid-red faceted stones and velvety-red cabochons from this mine set a new world standard for beauty, not just for red sunstone, but for any sun-stone. The opening of the mine also increased the supply of peach, salmon red-orange, red-green, green, blue-green, and bi colored and tricolored in combinations of yellow, red, and green. For the first time ever, there was a continuous U.S. supply of the more desirable colors in most of the calibrated sizes up to several carats needed by the jewelry industry. The largest faceted high-quality red sunstone is over 10 carats. The supply from this and other developed deposits will continue to be adequate for many years into the future.

Virginia 
The red Oregon sunstone is not the only gem feldspar produced in the United States that sets the world standard for quality. Amazonite from Amelia County, Virginia, also falls in this category. The deep, blue-green, translucent amazonite from the Morefield and Rutherford Mines is the finest in the world. The Morefield Mine is currently operated both as a commercial gemstone mine and as a fee-for-dig mine. The Morefield Mine ensures an adequate supply of high quality amazonite for cabochons, beads, carvings, and spheres. High-quality material from the Morefield is available in pieces over 0.5 kilogram in weight. The Rutherford Mine is open only a single weekend per year on a fee-for-dig basis. Colorado, California, and Montana also have deposits of amazonite. Of these locations, the mineral specimen-quality amazonite from Colorado is the best known.

Others
There is production of various gem feldspars from other deposits in the United States. Small to medium-sized pieces of light brown bytownite from Arizona and New Mexico can be faceted into 0.5- to 2-carat, eye-clean stones. In addition to the Oregon deposits of faceting-grade labradorite, gem-quality material has been recovered and cut from deposits in California and Nevada. Pale-yellow to colorless andesine is recovered from deposits in north-eastern Idaho. The material is found as eroded crystals more than 5 centimeters in length that can cut stones of more than 10 carats. Other States have reported deposits or finds of gem-quality feldspar. Currently, none of these deposits are commercially mined.

The production of gem-quality feldspar in the United States is increasing, as is the use of U.S. gem feldspar. This increased demand will improve prices for rough and thus provide the incentive for people to spend the time and money to mine the high-quality deposits in the United States. It is only a matter of time until additional deposits are mined on a commercial basis.