GOLD: karatage and gold colors

If you are not familiar with the gold business, you might be confused about gold, karatage and gold colors.

People who come in my jewellery shop, very often ask me the same questions about the gold items, in this article I would try to answer two of the most common questions and give more information about this metal:

“what does it mean the 750 that is stamped on gold jewellery?”

Gold is a noble metal but is not used pure in jewellery: it’s too soft to be worn without been damaged. A jewel made of pure gold would lose its original shape in short time, not mentioning the surface that would be full of scratches. 

This means that what we buy in shops are actually items made of an alloy of gold mixed with other metals. According to the percentage of pure gold inside the alloy, the item is stamped with its “karatage” which is a number (or symbol) that tells, out of 24, how much pure gold is present.

One karat (K or Kt) is 1/24^th part of a unit of gold. Therefore, if the gold is pure it is 24 Karat. 

Karat	Parts of pure gold out of 24	%  of actual gold in the item	stamp
24 Kt	24/24	99,99%	1000
22 Kt	22/24	91,67%	916
18 Kt	18/24	75%	750
14 Kt	14/24	58,5%	585
12 Kt	12/24	50%	500
10 Kt	10/24	41,67%	416
9 Kt	9/24	37,5%	375

Most Countries choose to make jewellery in 18 karats. If you have home some gold jewellery try to check it under magnification, somewhere you should be able to fine the inscription that tells which karatage it is. For 18 karats the inscription is either “750” or “18K” and usually they are followed by the logo or code of the manufacturer company that guarantees the karatage.

18K is an alloy where 6 part out of 24 are made of metals that allowed gold to be hard and tough enough to be worn and hold safely the gemstones.

If under magnification you can’t find any inscription, or the number you can see is not 750, there might be several explanation:

•            Maybe it was made in a Country, or a time, where the stamping was not mandatory.
•          If it’s stamped with 925, it might be silver.
•          A karatage different than 18K has a different stamping, check the chart above.

Nowadays is possible to see shops that sell items with any kind of karatage. The constant rise of the price of the gold made jewellery less affordable, so a low karatage imply less pure gold in the alloy, which means a lower cost.

“What is the difference between white and yellow gold? How can we have more gold colors?”

 Gold, naturally, forms only in the yellow color, but very often we see jewellery collection made of gold in different colors, how is that possible?

As we mentioned before, gold is not used pure in jewellery, but is mixed with other metals in an alloy that can modify the original yellow.

White gold is very appreciated worldwide; this alloy was invented after the First World War, there is a strong presence of silver and palladium that lighten the original color giving a whitish look.

The color is actually more grey than white: at the end of the making process, the item is coated with a thin layer of rhodium that gives the same look of platinum. The only problem: after some time, the layer can fade showing again the yellowish color underneath,  which can be covered with a new layer of rhodium.

Rhodium, palladium and iridium belong to the platinum group; the use of rhodium for platinum plating is very common in jewellery to achieve a whiter finish, especially to plate white gold and sterling silver.

In all this “colored” items the karatage is always guaranteed from the manufacturer company, so if in a 18K jewel we want to see different colors we just have to change the other metals of the alloy:

Yellow	75% pure gold	12-7% Silver	13-18% Copper
White	75% pure gold	25% Nikel, Silver or Palladium
Pink	75% pure gold	6,5-5% Silver	18,5-20% Copper
Green	75% pure gold	12,5% Silver	12,5% Copper
Red	75% pure gold	4,5% Silver	20,5% Copper

The percentage in these recipes may vary according to the final project, the factory or the rules of the Country where is made.

To make blue gold the process is slightly different: Iron is mixed in the alloy with gold and after is exposed to heat to oxidize the iron atoms on the surface, which produce a bluish coloration.  

Malachite

Malachite is a green opaque gem. It crystallize in the monocline system.

The name comes from the Greek MALAKE’, that means mallow, of which it has the color of the leafs. Its main characteristic is the curved bands pattern in several green tones, from light to dark one. the color is given by the main component, copper, in the past it’s used to be found in deposits near to this metal. Thanks to its colour is considered the gem symbol of hope, love and faith.

 

The hardness is 3 and a half on the Mohs scale and, because sometimes it is found in big blocks, is quite often carved to get cameos, art crafts and statuettes.

The toughness is quite low, that’s why it must be handled with care. It can be ruined by chemicals, especially acids. It is also quite sensitive to heat and hot water.

In the past the main source of Malachite used to be Russia, but now new deposits are in Africa, Australia and USA. The best samples come from Shaba, Kenia.

Sometimes is found along with the stone Azurite; the match create a stone called Azurmalachite.

In nature it is found in microcrystalline clusters consisting of fine needles. The exact scientific term that describes its natural form is Botroydal, which refers to its irregular nodules growth in concentric layers, which, once cut, show the banding pattern that characterizes it.

Historically, it has always been a popular gemstone since the ancient Greeks and Romans, who used it for carvings and ornaments that we can still see today. In Latin culture it used to be associated with the Goddess Venus, they believed it can help to reunify lovers and also console those who have lost their loved one.

Was also used as an amulet to protect the owner from danger and especially to protect children, since the time of Solinus it was suggested to put it directly in the cradle of infants; also, to be placed near the woman womb to support conception and childbirth. According to ancient beliefs it  could be pulverized and dissolved in milk to help with heart disease, or mixed with honey and applied to wounds to promote healing.

When  in the 18th century rich deposits were found in Russia, the mineral became the favorite ornamental stone by the court of the Tsar. An example of this are the great pillars of the church of St. Isaac in Leningrad.

Malachite is considered a semi-precious gemstone and maybe this is the reason why nobody never created a synthetic version, but in the market there are many other cheaper alternatives, such as plastic, glass or dyed agate.

The only type of treatment that is applied to enhance the appearance is the impregnation with mineral oils to make the surface shiny and smooth.

Ruby

RaffaelloSanzio: Portrait of Maddalena Doni

The name Ruby comes from the Latin Ruber, which means “red”. Medieval Europeans wore ruby to succeed in Love, health and wealth.  Because of its color has always been associated with passion, love and beauty. It is the world most famous and best-loved red gem. 

 

Is the most important colored stone in the market and is the red variety of the gem species Corundum. 

The quality of a ruby is determined by different factors, the most important is the color.  The finest Ruby has a vivid red color with a light purple hint. In its purest form, the corundum is colorless, but if some trace elements stick into the crystal’s structure, the mineral can have some change in color. In the case of Ruby, the trace element that affects the red color is the Chromium. The saturation of the color depends of the quantity of Chromium present in the Corundum.  Because the Chromium causes fluorescence, the best ruby must have also some traces of Iron, which inhibits the fluorescence. 

Its hardness is 9 on the Mohs scale, is the hardest stone on Earth after the Diamond.

Historically the best quality color ruby comes from the Burmese source, which is described as “pigeon’s blood”.  Even though the origin of a Ruby is important to guarantee the quality, not only Myanmar produces high quality gems, any ruby source can produce both high and low quality stones.

The major sources of Ruby are in Asia: Myanmar, Thailand, Sri Lanka, Cambodia, and Pakistan.

 

There are mines also in Africa: Madagascar, Kenya, Malawi and Tanzania.

 

Natural Rubies are very often included, and is also difficult find it in big specimen. It’s rare see an eye clean ruby in exceptional size.  The environment where the ruby forms makes it difficult, for the crystal, to grow.

But the story is different for the sapphire, the brother of Ruby; the blue stone is easily found in bigger size and less included.

 

Treatments are widely used to enhance the beauty of the Ruby, and the heat treatment is now so common that is accepted in the trade, actually in every gem market it is assumed that all rubies are heated, unless there is a certificated that can prove it. Heat has multiple benefits; it works in both color and clarity. Any brown shades, that lower the quality, disappear to bring out the red color; while also the inclusions become less visible. Other used treatments, less accepted from the trade, are dyeing and fracture filling. 

Can happen sometimes, to see in the market the star ruby, the phenomena, called asterism,  is caused by the needle-like inclusions into the stone, if is cut into a cabochon the light is reflected in a 6-rays star. Those gems are highly priced and very rare.

Ruby and star ruby are both the US birthstones of July.

Ruby is one of the earliest colored stones to be imitated, with both natural and synthetic materials.  Glass is the most common material used as imitator. Other material are: red spinel, garnet, CZ, and dyed corundum.

Red Spinel

 

Garnet

 

CZ - Cubic Zirconia

In late 19teen century, some scientist introduced in the market a new kind of rubies manmade in lab. They found the way to synthesize in the lab what Mother Nature makes in million years. The process is quick and cheap.  Those synthetic rubies are now easily affordable, not expensive and look really good to the untrained eye. But don’t worries, the gemologist has the knowledge to distinguish between those and a natural one, if you want to buy a jewel with Ruby, always ask for a certificated, just to be sure!

Turquoise

Turquoise is one of the most ancient gems. Archaeologist found royal jewellery made in turquoise older than 7000 years ago. Chinese artisan carves it since 3000 year ago and is the national gem of Tibet. Is considered a stone that guarantees health, fortune and protection from evil.

In southwest US, native American tribes was used as medium of exchange, and also for jewellery and amulets. The apaches believed that turquoise enhanced a warrior’s accuracy.

The gem’s name comes from the French expression pierre turques, Turkish stone, because the gem first arrived in Europe from Turkish source.

The stone can be translucent to opaque, and has a unique light blue color. The blue is not always uniform, it’s actually often mottled and with dark splotches. It might also have veins of matrix.  The quality is judged on color, texture and absence of matrix.

The blue color is caused by the presents of copper, while the iron makes the stone look more greenish and lower the quality.

The color can be unstable, it can darken, fade or became green, depending on the other foreign substance that are absorbed from the stone.

The color changing is caused by the porous surface, that can also affect the durability. On the Mohs scale its hardness is 5 to 6, perfect to be worn but with care.

Matrix veins are often visible, even though cutters always try to fashion the final gems without it.

Turquoise gem is not so rare, but is very difficult find really high quality (that are highly priced on the market). Treatments are very common either to enhance beauty and to make them resistant to foreign agents.

Wax impregnation is the most used treatment. The wax improve luster, seal the pores and avoid contact with skin,  perfume and water; if you have any jewel in turquoise please pay attention  when you wear it: contact with sweat or cosmetics may damage them irreversibly.

Gem with light color are dye impregnated , but this technique is actually not used often, because the result looks unnatural and it doesn’t last.

The first historical source of Turquoise is Persia, but now the most important mines are in southwest US and China, where comes the specimen of highest quality.

Like any other gem , even turquoise has its synthetics and imitations. we said that the market is full of turquoise, but not of high quality samples; which is why in the ’80 was created the Gilson synthetic Turquoise. The material is produced by a ceramic method and has an artificial appearance, actually it was never widely available on the market.  Instead, the imitation are very common in the market, both natural and synthetic. The natural gems that most resemble the turquoise are: Variscite, dyed howlite and chrysocolla. Among the most widely synthetic used are glass and plastic.

dyed Howlite

Variscite

Chrysocolla

Lapis Lazuli

Man first discovered lapis lazuli almost 7,000 years ago. It has been found in ancient civilizations such as Egypt, Persia, Greece, Rome and the Middle East. 

It’s an easy, workable stone, so it’s not used only in jewelry, but also carving, mosaics, ornaments. It is also processed to make blue pigments for make-up and painting.

In Egyptian culture, this stone has an important connection with religion. Archaeologists found abundant amulets and talismans made in Lapis.

Its vivid blue color made it an important component of some European masterpieces. The frescos of some important medieval churches in Italy were made using the blue pigment made from lapis lazuli.  It is pretty impressive if you consider that at that time the Lapis was really expensive and was available only from one far source: Afghanistan.

Lapis lazuli is not a gem, but a rock, which means that it is an aggregate of several minerals, including but not limited to: Lazurite, Calcite and Pyrite. The quality of the stone is determined by the percentage of these three minerals. For example, if the calcite matrix is too present in the stone, it looks white and therefore less appealing.

Lazurite

pyrite

calcite

The presence of the pyrite flakes makes the stone look like a sky full stars, and this is the reason why people believed that the stone unleashed peace and  relieved stress. Kings and queens used it in the past as a powerful status symbol; it was considered a piece of heaven given to men by the gods.

The top quality color is a violetish blue with no-visible calcite and some gold looking pyrite flecks. The presence of an excess of calcite or pyrite can lower the quality of the stone and make it look greenish and less valuable. Its hardness is 5 to 6 on the Mohs scale.

It can be found in different shades of blue, depending on the other minerals in the aggregate.

On Earth, there are not many Lapis mines: the finest quality is found in Afghanistan, the oldest source, but it is found also in Chile and Russia.

Because fine-quality Lapis stones are highly priced, treatments are common, but not stable, which makes the treated stone less valuable than untreated one. The dyeing treatment is used frequently; it’s used to enhance the blue coloration and cover the white coloration caused by the calcite but can be rubbed away with acetone. Usually after dyeing, it is also impregnated with wax to protect the stone and make it appear shiny.

Gilson 

Lapis imitations are plenty and are old as lapis itself. Glass, plastic and ceramic are very common in the market, they are manmade at a low cost.  Another manmade material created just to imitate Lapis is the Gilson Imitation Lapis Lazuli, which apparently looks just like the original, but is actually very porous. Is Its hardness is 3 on the Mohs scale and the luster is dull. The color is perfectly even and the pyrite flakes appear like sprinkles on the surface. Other substitutes are Chalcedony and sodalite.

Until the 1950s it was one of the December birthstones, so it can be used as a good alternative to Tanzanite and Turquoise for people who were born in that month.