Interesting facts about Citrine

Interesting facts about Citrine

                                            

Color ranges from pale yellow, to medium yellow, to medium-dark orange-yellow, to root beer color.  Most Citrine on the market is actually a brownish variety of an  Amethyst that has been heat treated to produce the bright yellow colors and darker stones with reddish tints.  The pale yellow color stones are typically natural untreated stones.  As the color ranges into medium-dark orange-yellow to Root Beer it is called Madeira Citrine.  Citrine is heat sensitive, excessive heat could turn the stone colorless, an abrupt temperature change may fracture the stone.  Chemicals that could attack this gem are hydrofluoric acids, ammonium fluoride, and alkalies. Sources for this gemstone are Brazil, Bolivia, and Madagascar.

Citrine, a member of the Quartz mineral family, is a Quartz whose color is probably caused by Iron traces. 

Citrine's name is derived from the French Citrine for lemon, it might also come from the Latin ‘citrina’ that means yellow.

The term “citrine” was formally used only after 1556. Up until then, the stone had been known simply as yellow quartz (sounds much less glamorous, right?).

It is nearly impossible to tell cut citrine from the more expensive yellow topaz with the naked eye. Their main difference between the gems is in hardness, so have your gem testing tools handy.

Most citrines are actually heat treated amethysts or smoky quartz. The natural yellow colour of citrine is rather pale, but the heat-treated stones have deeper colour, sometimes with a slight reddish tint. I hope  you did not get disappointed that the favourite citrine jewel of yours has in fact a heat-treated gem.

The largest citrine weighing 2258 carats originates from Brazil and is displayed at the Smithsonian in Washington. The gem is extremely large considering that the typical size used in jewellery isunder 25 carats.

From pastel yellows, to wonderful orange hues Citrine will fl atter many fashion-conscious pallets. Yellow hues are striking and are 

symbolic of the sun, health, and vitality. Since it is plentiful in nature, it represents an incredible value for the person who appreciates. 

its colour. It is available in a wide range of shapes and sizes, and it is a favorite choice of many jewelry designers.

Brazil is the main supplier of citrines. However, they are also found in Madagascar, Zambia, Namibia, Ural Mountains in Russia and in most countries where amethysts are found. Now I have another good reason to visit those countries.

Citrines can scratch relatively easy and need to be looked after. One should store citrine jewellery out of direct sunlight as the colour can sometimes permanently fade.

The Romans were thought to be the first to wear citrine, mostly as cameos or intaglios. The gem re-discovered its popularity during the Romantic period as it enhanced the beauty of gold jewellery. Nowadays, citrine jewellery is widely worn and loved, so the gallery with the latest honey-coloured jewels is coming up shortly.

Citrine is the birthstone for November and the traditional anniversary gemstone for the 13th year of marriage. 

A tremendous amount of citrine that is available on the market today is heat-treated amethyst. Natural citrine is much lighter than the heat-treated material which is dark orange-brown to reddish-brown in color.

Chemical Composition:  Silica (Silicon Dioxide) SiO2

Colors / Varieties:  Yellow, Golden

Crystal System / Forms:  Trigonal System

Hardness:  7

Specific Gravity:  2.63 - 2.68

Cleavage / Fracture:  No distinct cleavage / conchoidal to uneven fracture.

Optic Character:  Anisotropic, D.R.; Uniaxial positive. May exhibit a bull's eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).

Lustre:  Vitreous.

Refractive Index / Birefringence:  1.544 - 1.553 / 0.009. Range: 1.535 - 1.560

Pleochroism:  Yellow / Slightly paler yellow

Dispersion:  0.013

Magnification:  Liquid and two phase, color zoning, negative crystals, zebra-stripe finger prints (structural), crystal inclusions, green fuchsite mica flakes / platelets in aventurine quartz, rutile / tourmaline needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.

U.V. Fluorescence:  Variable.

Spectrum:  Not characteristic.

Cause of Color:  Trace of Iron (Fe3+)

Treatment (Enhancement): 

Coating: all colors, thin film of gold on colorless will give a blue color (aqua aura).

Colorless impregnation: with oil or resin to improve clarity.

Colored impregnation (dyeing): all colors, quartzite

Heat Treatment:

Light yellow to deeper yellow

Irradiation:

Citrine to amethyst

History of the Diamond as the Birthstone

History of the Diamond as the April Birthstone

Diamonds are a thing of beauty and the ultimate gift for a loved one. Thought to be one of the hardest substances on the globe, diamonds date back billions of years. The diamond is the traditional birthstone of April and holds significant meaning for those born in that month, thought to provide the wearer with better relationships and an increase in inner strength. Wearing diamonds is purported to bring other benefits such as balance, clarity and abundance.  It's also symbolic of eternal love, and those fortunate to call April the month of their birth will enjoy the following history behind this rare gem.

Definition of Diamond, the April birth stone: The word Diamond is derived from the Greek word "adamas" which means "hardest metal" which reflect the property of the stone. The diamond is a precious stone or gem excelling in brilliancy and beautiful play of prismatic colors, and remarkable for its extreme hardness. This gem is a member of the Diamond (Carbon) family. It is the hardest substance known. The diamond as found in nature (called a rough diamond) is cut, for use in jewelry, into various forms with many reflecting facets, by which its brilliancy is much increased. The crystal structure of the Diamond, the April birthstone, is Isometric.

Since ancient times, diamonds have been admired objects of desire. Formed one hundred miles beneath the Earth’s surface over a billion years ago, diamonds are the hardest gem of all. Diamonds have a long history of folklore; some of which say diamonds were created when bolts of lightning struck rocks, and others said the gem possessed healing powers. For centuries, diamonds have been adorned by women and men and regarded as the ultimate gift and a symbol of eternal love.

Today, diamonds are still admired all around the world. Until the middle of the twentieth century, there was no standard by which diamonds could be evaluated. GIA created the first, and now globally accepted standard for describing diamonds: Color, Clarity, Cut, and Carat Weight. Today, the 4C’s of Diamond Quality are the universal method for assessing the quality of any diamond, anywhere in the world.

From the stunning display of stones flashed by movie stars at awards ceremonies, to the modest engagement ring of a young bride, the diamond continues to enjoy great popularity among those of all status.   Actress Zsa Zsa Gabor summed up this universal love of diamonds when she remarked, "I have never hated a man enough to give his diamonds back."

Interesting facts about Aquamarine

 Aquamarine ranges in color from light to deep greenish-Blue through blue-Green. The richer the color, the more costly the gem is per carat.  Aquamarine achieves its Blue to greenish Blue color from Iron in the ferrous state.  Its name is from two Latin words meaning "water" and "sea".  Aquamarine amulets were thought to render sailors fearless and protect them from adversities at sea, especially if the stone was engraved with a Poseidon on a chariot. The stone was symbol of happiness and eternal youth, and according to Christian symbolism, it signified moderation and control of the passions to it's owners.

Aquamarine is a member of the Beryl (BARE-ul) mineral family, along with Emerald, Golden Beryl, Goshenite, Bixbite and Morganite.  Brazil is the principal source for Aquamarine. More than 80% of the the Brazilian Aquamarine comes from an area around Teofilo Otoni in the western part of Minas Gerais.  Aquamarine is also found in China, Pakistan, Afghanistan, Madagascar, Malagasy Republic, Nigeria, Zambia, U.S. in Maine as well as Idaho and California, and the former Soviet Union in the Ural Mountains, as well as Transbaikalia and Siberia.  Aquamarine is generally not heat sensitive, but chemicals that could attack this gem are hydrofluoric acids.


Aquamarine gemstones with intense colors are becoming very scarce, and their price has increased substantially.


Top colors according to the GIA color grading system have 2-4 tones, with 1-4 saturation.

Typically Aquamarines are greenish when mined and cut, the sky blue shades are produced by heat treating the greenish-yellow, greenish, and even brownish beryl gems, so you should assume that all of the intense colors of Aquamarine may have been heat treated.  The heat treatment process is a permanent process that does not adversely affect the performance and durability of the gemstone, so there's no need to worry when you see this enhancement disclosed. Aquamarine should not be cleaned in an ultrasonic or steam cleaner.

Definition: Aquamarine (beryllium aluminum silicate) is a blue semi-precious stone in the beryl family and is closely related to the emerald. It is a transparent, pale green variety of beryl and is often used as a gemstone or birthstone. The color of the aquamarine is due to trace amounts of iron impurities in the beryl structure

•  The origin of the name Aquamarine is derived from the Latin words "aqua marina" which means "sea water" reflecting the color of the crystals.

• The most common color of Aquamarine is natural blue. The Aquamarine is the birthstone of the month of March. 

• Aquamarine are also the Wedding anniversary gemstone and are given as gifts for the 19th year of marriage

• Aquamarine come in a vast range of natural colors and shades from blue to blue-green to sea-green

• The color is mainly determined and caused by the gem's selective absorption of certain wavelengths of light called the body color.

•  The colors of the Aquamarine gem consist of three components, the hue, tone and saturation:

1. The Hue which gives the first impression of the color
2. The Tone which relates to the the lightness or darkness of the color of the Aquamarine ranging from light to medium to dark.
3. The saturation which relates to the intensity of the Aquamarine's color, which is measured from dull to strong to vivid

• The biggest Aquamarine stone was found in Brazil in 1910 and weighed 243 pounds. It was cut into smaller stones which yielded over 200,000 carats

• All gemstones, including Aquamarine, are graded by color, cut, clarity and carat weight

• The tradition of giving gifts of birthstone jewelry containing Aquamarine dates back many hundreds of years
• There are specific birthstones and gemstones, such as Aquamarine, which are traditionally given according to the month that a person is born

• Pure crystals of Aquamarine are used as gemstones. The crystal structure of Aquamarine is Hexagonal

Interesting facts about Amethyst

Interesting facts about Amethyst

Amethyst is the most highly valued member of the Quartz mineral family, and it has Purple hues that vary from very pale to dark.  The medium-dark to dark tones are considered the most desirable and of course are quite a bit more expensive than the lighter tones.  The finest gems usually come from the Elahara mines in Sri Lanka, and the Pau d'Arco mines in Brazil.

These fine gemstones are deep dark and rich in color, with all the fire in sparkle you would want to see.  Most Amethysts on the market are relatively free of inclusions. Amethyst is heat sensitive, excessive heat could turn the stone colorless, or it may produce a Citrine or green color, whereas an abrupt temperature change may fracture the stone. Chemicals that could attack this gem are hydrofluoric acids, ammonium fluoride, and alkalies.

Amethyst is produced in abundance from the state of Minas Gerais in Brazil where it occurs in large geodes within volcanic rocks. Many of the hollow agates of southwestern Brazil and Uruguay contain a crop of amethyst crystals in the interior. Artigas, Uruguay and neighboring Brazilian state Rio Grande do Sul are large world producers exceeding in quantity Minas Gerais, as well as Mato Grosso, Espirito Santo, Bahia, and Ceará states, all amethyst producers of importance in Brazil.

It is also found and mined in South Korea. The largest opencast amethyst vein in the world is in Maissau, Lower Austria. Much fine amethyst comes from Russia, especially from near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks. Many localities in south India yield amethyst. One of the largest global amethyst producers is Zambia in southern Africa with an annual production of about 1000 tonnes.

Amethyst occurs at many localities in the United States.[11] Among these may be mentioned: the Mazatzal Mountain region in Gila and Maricopa Counties, Arizona; Red Feather Lakes, near Ft Collins, Colorado; Amethyst Mountain, Texas; Yellowstone National Park; Delaware County, Pennsylvania; Haywood County, North Carolina; Deer Hill and Stow, Maine and in the Lake Superior region of Minnesota, Wisconsin, Michigan, and Ontario in Canada. Amethyst is relatively common in Ontario, and in various locations throughout Nova Scotia. The largest amethyst mine in North America is located in Thunder Bay, Ontario.

Amethyst mines can be found in Canada, Brazil, Argentina, Bolivia, Russia, India, Zambia and other African countries. The largest of these mines are in Brazil. Unfortunately, care must be taken when buying Amethyst stones as it is possible to come across some fake stones. Luckily, these fakes can be spotted as the stone will look flawless and too perfect. Natural occurring amethysts will have some flaws inside of the gem indicated that it must have originated in a mine as opposed to being grown.

Up until the 18th century, amethyst was included in the cardinal, or most valuable, gemstones (along with diamond, sapphire, ruby, and emerald). However, since the discovery of extensive deposits in locations such as Brazil, it has lost most of its value.

Collectors look for depth of color, possibly with red flashes if cut conventionally. As amethyst is readily available in large structures the value of the gem is not primarily defined by carat weight, this is different to most gemstones where the carat weight exponentially increases the value of the stone. The biggest factor in the value of amethyst is the colour displayed.

The highest grade amethyst (called "Deep Russian") is exceptionally rare and therefore, when one is found, its value is dependent on the demand of collectors. It is, however, still orders of magnitude lower than the highest grade sapphires or rubies (padparadscha sapphire or "pigeon's blood" ruby).

Interesting facts about Alexandrite

Interesting facts about Alexandrite

                                                                                           
Alexandrite changes color from green in daylight to red in incandescent light. The first time you see it, it is hard to believe your eyes!  Alexandrite was first discovered in Czarist Russia, in the Ural mountains, on April 3 1834, when several crystals were found in Tokovanya deposits.  The name “Alexandrite” was coined by mineralogist Dr. Nordenskjöld (Finnish by birth but he worked for the Russian Czar). Since the old Russian imperial colors were red and green it was named after Czar Alexandr II on the occasion of his coming of age in 1842.  Genuine Alexandrite is one of the world's rarest and most coveted Gemstones, it's so rare that most people have never even seen a genuine Top Quality Alexandrite.

Alexandrite is in the Chrysoberyl mineral family and is one of the most fascinating gemstones throughout history. Alexandrite is the variety of Chrysoberyl that displays a change-of-color from green to red. A distinct color change is the primary qualification for a Chrysoberyl to be considered alexandrite. Although alexandrite is strongly trichroic, its color change has nothing to do with pleochroism. Instead, like all other color-change gems, it results in a near-equal transmission of the blue-green and red portions of the spectrum, coupled with strong absorption in the yellow. Thus its color is dependant on the spectral strength of the light source. Incandescent light is strongly tilted to the red end, thus causing alexandrite to appear reddish. Daylight, is more equally balanced. Since our eyes are most sensitive to green light, the balance is tipped to the green side. The strength of the color change is related to the difference in the areas of transmission, relative to the absorption in the yellow. The greater the difference, the stronger the color change.  The color is actually caused by Chromium.

A color change occurs in very few gemstones. For alexandrite, the quality of the color change is paramount. While the holy grail is a gem whose color changes like a traffic light from green to red, such a stone has yet to be found. In fine examples, the change is typically one from a slightly bluish green to a purplish red. The quality of color change is often referred to by dealers in a percentage basis, with 100% change being the ideal. Stones that display a change of 30% or less are of marginal interest and are arguably not even alexandrite. Significant brown or gray components in either of the twin colors will lower value dramatically.

The natural color change in Alexandrite ranges from various shades of Green (blue-greens, kelly-greens, olive-greens, teal-greens, etc.) when the gem is under "fluorescent" lighting, or in natural outdoor light "in the shade" (not direct sunlight)... then the color changes to various shades of Red (burgundy-reds, purplish-reds, reddish-purples, violetish-purples, amethyst, etc.) when exposed to light in a room with only incandescent light, tungsten light, or candlelight (a flashlight will bring out the color change too).

To get the maximum color change effect the gem is best viewed in a dark room with no outdoor light at all, then turn on a fluorescent light to see the green shade, then turn on a standard incandescent light bulb with the fluorescent light off to see the reddish shade.  You can cycle the lights back and forth to see the distinct color change.  Alexandrite will normally darken when taken out into direct sunlight, since it is subjected all the colors in the spectrum,  also if subjected to several types of lighting sources at the same time the colors may twinkle with both the red and the green color bouncing around different facets in the stone.  When the primary color has a bit of brownish in with the green, this is due to the bleeding of the two colors.  Typically this is referred to as the percentage of color change.  The higher the color change percentage, the more expensive the gem.

Tanzanite by Peter Bancroft

Tanzanite by Peter Bancroft

Merelani Mine, Arusha, Tanzania

Editor’s Note: We are pleased to reprint this selection from Peter Bancroft’s classic book, Gem and Crystal Treasures (1984) Western Enterprises/Mineralogical Record, Fallbrook, CA, 488 pp.

Zoisite was the mineral name for a series of opaque to translucent varieties, of which only two are used as gemstones: massive pink colored thulite and a brilliant apple-green zoisite (which also occurs in grayish-white, gray, yellowish-brown, and greenish-gray). Named for Czechoslovakian Baron Zois von Edelstein, zoisite never really achieved prominence as a gem material.

Then in March 1966, an Arusha tailor named Sousa discovered a deep blue gemstone at a place later known as the Merelani mine. Using sophisticated equipment experts found that the gem was not “sapphire” as suspected, but an astounding variety of zoisite. Tiffany and Company, in a vigorous sales promotion, named the stone “tanzanite” after the country of origin, and the name has since been generally adopted.

The Merelani deposit was difficult to reach in the Usumburu Mountains that border the Umba Valley of northern Tanzania. This region, a vast arid plain broken by hills and small mountain peaks, is inhabited by the pastoral Masai people. The Merelani lies between two landmarks just south of the Kenya border – Mt. Kilimanjaro and the Olduvai Gorge made famous by paleoanthropologist Louis Leakey. The nearest town of any consequence is Arusha, about 50 kilometers to the northwest.

Small landing field near Mt. Kilimanjaro. Photo: Peter Bancroft

In 1975 the author met A.H. (Brick) Stange who provided a pilot and a single-engine airplane for the flight from Nairobi to Arusha. We planned two trips – first a visit by plane to the ruby mine near Longido and the second in jeeps to the tanzanite mine in Merelani.

One of the original claim holders who lost his property when the Tanzanian government nationalized the mine. Photo: Edward Swoboda (1967)

The trip to the ruby mine was uneventful except for a painful bite from a tsetse fly. Even though my seat belt was fastened, I jumped hard enough to bang my head on the cockpit ceiling and the pilot lectured me on how dangerous it was to make such a violent movement while flying at a low altitude. Just as he concluded his rebuke, a fly bit him on the neck. There was an immediate plethora of epithets, somewhat superior to mine, but no more lectures! Upon reaching the mine we noticed a great wall of schist marked by green streaks of zoisite. Closer inspection revealed bright red, nearly opaque 2 to 4-centimeter ruby crystals imbedded in the zoisite. Large chunks of the red and green material were blasted loose for shipment by truck to Arusha.

A few days later our party left in two jeeps for Merelani. Our route passed through a brushy countryside dotted with trees, and from time to time, we passed Masai villages and herds of Brahman cattle tended by one or two small boys. Upon reaching the mine, we were admitted by armed guards. The tsetse flies were replaced by numerous non-biting flies that landed upon any part of the body which exuded moisture – the corner of the eye or mouth and exposed parts when relieving oneself. They were a constant irritant and a real nuisance.

Tanzanite Size: 220 carat faceted, cushion cut stone. Locality: Merelani; Collection: Edward Swoboda; Photo: Harold and Erica Van Pelt

Guards armed with rifles and shotguns were conspicuously posted at vantage points about the workings. Digging was by the open-pit method, with a few veins being explored by short tunnels. A small padlocked iron box sat nearby. When a miner discovered a crystal or segment, he was to drop it into the box. However, the chances are that many were swallowed or thrown into a bush for recovery later. Miners were paid next to nothing, and this provided the incentive to high-grade stones.

Masai family near Merelani mine. Photo: Peter Bancroft

Tanzanite crystals occur sparsely in a heavily metamorphosed zone of tough rock. They are prismatic, heavily striated and often possess good terminations. The mineral is fairly soft (6 to 6.5 on Mohs’ hardness scale) and fractures rather easily. It is common to expose a fine crystal deeply imbedded in rock, then watch it break into small pieces during removal. The search for crystals is accomplished with picks, iron bars, shovels, and compressed-air jackhammers, a system that takes a high toll of fine gems. Clean stones which will facet 2-carat or larger gems are quite rare, and days pass between discoveries.

Miner examining tanzanite crystal. Photo: Edward Gübelin

Tanzanite occurs in colors of gray, brown, violet, blue, reddish-purple, and tints of green. The gem is of interest because of its exceptional pleochroism. Rotated in different directions a single stone will exhibit three outstanding changes of color, from reddish purple to blue to deep purple. When placed in ovens and heated to 620°C, the reddish purple crystals alter to a rich violet-blue; some turn to a deep blue resembling sapphire. But heating reduces pleochroism.

Miner breaking tanzanite from mother rock. Photo: Edward Gübelin

Tanzanite in large flawless sections appeared with some regularity during the early days of mining. Lovely gems of 20 to 50 carats were not unusual. One of the largest stones, a splendid faceted flawless gem weighing 122.7 carats, is part of the Smithsonian Institution's collection.

Following tanzanite vein into hill. Photo: Peter Bancroft

When first mined, fine stones could be purchased in Tanzania, Kenya, and the United States for as little as $20 a carat. By 1984 clean gems wholesaled at better than $1000 a carat, an indication of increasing interest in a gem already in short supply. In 1978 a massive cholera epidemic struck northern Tanzania and interrupted production of rubies, garnets, sapphires, chrome tourmaline, chrysoprase, and tanzanite. In addition, most of the known gem deposits were playing out. As a result, decreasing production has caused a scarcity of quality gemstones and a rapid rise in their value.

Magnificent untreated tanzanite crystal. Crystal: Bill Larson collection; Photo: Jeff Scovil

Miners working a Merelani vein under watchful eye of armed guard. Photo: Peter Bancroft

Screening day’s output of tanzanite. Photo: Peter Bancroft

Rhodium plated jewelry - Yes or No??

Many buyers ask why .925 sterling silver is rhodium plated.  

The improved appearance of rhodium plating was initially noticed because of its extra beauty to the naked eye, but as the points below will show, it not only enhances the beauty of silver, it helps ensure the longest-lasting, most durable finish for your jewelry. The result:  stunning jewelry that stays stunning much longer than un-plated  jewelry.

Rhodium is considered best in class of all the platinum metals family for resistance to corrosion and tarnish, and is the most tarnish resistant member of the platinum metals group.  Rhodium will also remain bright and shiny under most environmental conditions. Not surprisingly, of all the metals in this group, rhodium is by far the most expensive to utilize.  Rhodium is nearly 4x as expensive as platinum, and 9x as expensive as gold.

As a protective coating, rhodium finishes virtually eliminate any chance of surface discoloring even when exposed to extreme environmental conditions under normal temperatures. The surface hardness of plated rhodium is also the highest of the precious metals, and is thus highly resistant to incidental scratching and abrasion. Rhodium plated finishes are best in class for wearability among the precious metals.  In reality, silver is shinier than rhodium, but silver is also a softer and more malleable metal, and can be scratched and dented more easily. So jewelers may choose to rhodium plate some jewelry items to make them more durable.

Additionally, rhodium is hypoallergenic. White gold and even sterling silver normally employ nickel to provide the whiteness of white gold, however, approximately 20% of women will have some form of allergy to nickel. Thus, by rhodium plating, a possible allergen is removed from contacting your skin.  The one exception to that is on stud earrings. Because the nut of the earring drags along the post, the post will not remain plated for long. In those instances where nickel may be a problem, we recommend using plain sterling or platinum earrings.

Rhodium plating can dramatically increase the long term durability, brightness and luster of the finish. Because it is a plating, it will wear off over time.   How soon depends on how much wear and tear the jewelry item takes.  When the rhodium plating begins to disappear from a silver or platinum jewelry item, it will be much less noticeable because the underlying metal will look gray, not yellowish.  However, virtually any jeweler can quickly re-plate your jewelry, usually for around $45, to bring back the original shine and luster that rhodium can provide.

Rhodium plating should not be done without training, appropriate safety equipment, good ventilation, and concern about the workers’ health and safety.  Some manufacturers have become concerned about how this process can harm the environment if improperly used or disposed of, and have begun to use alternatives such as Argen.  Others use  processes which keep using  the same bath for 5 to 10 years and more, with simple continuous filtration systems and good cleaning procedures.  It is not very expensive, and certainly cheaper than starting a new bath every time the bath gets contaminated.  We applaud those who manufacture or use rhodium plating systems safely, and will support those who chose to use alternatives which keep all our systems clean and safe.

Keep in mind that a rhodium plate will not eliminate dents and scratches, it will only make the jewelry more shiny and bright (some would even say more expensive looking!). Remember to ask your jeweler if he or she is planning to clean and polish your jewelry before its re-plated, if not then ask for it. The smoother the jewelry item is when starting, the more shiny it will look after the rhodium plating process.