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Sunday, January 23, 2011

The King of Stones the Ruby

Rubies are a red colored form of corundum a form of aluminum oxide with the chemical formula of Al2O3.  This is the same mineral that produces sapphires that are found in blue and virtually every other color except red that color is reserved for the ruby only.

A ruby crystal on calcite.
Photo by Rob Lavinsky

To be a ruby the mineral corundum has to be colored pink to blood-red where the color is caused by a minute quantity of the element chromium.  The red color gives it its name “ruber” that is the Latin word for red making for an easy move to “ruby” in English.  In the trade ruby is considered one of the four precious stones the others are diamond, emerald and diamond.  Some people consider natural pearls to be the fifth precious stone although the owe their origin to natural processes produced by a living animal.

Hands down a good ruby exceeding 10 carets in weight is the most valuable of all gems.  Like any other gem the prices for rubies are mainly determined by their color, so that the most valuable stones are those that are bright red the so-called pigeon blood-red that commands a huge premium price when compared with any rubies having a similar quality.  The same as grading diamonds the next criteria for rubies is clarity where a clear stone will also command a premium.  However, a ruby containing needle like inclusions of the mineral rutile will often produce a star on a ground specimen that commands a high price as a star ruby.  Most rubies have been heat treated causing the rutile inclusions to vanish indicating that the stone has been treated.  The final quality that can determine a rubies price is cut and its weight in carets.  A recent price quoted for a ten caret pigeon blood-red stone was $24.000 per caret.  This meant the stone had a wholesale value of $240,000.
A cut pigeon blood ruby.

Rubies are most often found in suture zones, a good example is the ruby deposits found in the Mogok Valley of Myanmar where the rubies are found in an altered limestone that when it was laid down received a clay containment that was rich in alumina.  When the limestone was metamorphosed to marble the alumina that is aluminum oxide was converted into corundum.  The addition of a small amount of chromium ions caused the corundum to produce rubies.

Rubies are often a component of marble a good example of this is the occasional ruby crystal found in Franklin, New Jersey.  Sometimes marble undergoes further metamorphism into a rock called amphibolite that also can contain rubies as it does in a newly discovered deposit in southwest Greenland.  Although the stones in this deposit are small they make up for it by being almost perfect pigeon blood-red.

One of the necessary criteria for the formation of all rubies and sapphires it that they form in a rock that is starved of silica;  sometimes these stones are found associated with deposits of ultramafic rocks that is the apparent association where many rubies have been found in the gravel of the Cowee River in Franklin, North Carolina.

This is a place where the general public is invited to find their own rubies by washing, and separating the rubies from gravel.  Most of these stones are heavily contaminated with impurities, but many make some very fine gems.  Occasionally a stone is discovered that rivals the best stones produced in Myanmar.  Rubies are also found in several other parts of the world notably in Madagascar and Sri Lanka where they are found in gem bearing gravel.

Friday, January 21, 2011

Diamond the Hardest of all Gems

Photo by Subarcticmike
The very name for the diamond is “adamant” meaning roughly that it is indestructible.  The gem we call diamond was known to the ancients when practically the entire earth’s supply came from the region around Golconda, India.  This region in India supplied most of the diamonds in the world until the discovery of alluvial diamonds in Brazil in 1729.  The diamond fields of Brazil produced most of the world’s diamonds for almost a century until diamonds were discovered in South Africa in 1867.  This led to the discovery of diamonds in an ultramafic rock that was named kimberlite after the South African town of Kimberly where the new rock was discovered.

A diamond crystal in a matrix of kimberlite. USGS

The other source of diamonds during antiquity was from western Borneo where they are found in alluvial deposits of Eocene age.  Most of the stones found in these deposits are small and of poor quality.  These diamonds are still being mined mainly by artesianal methods under primitive methods.  To this day nobody has ever discovered the primary source of these diamonds.

During the post World War II period extensive deposits of diamonds were discovered in the Soviet Union by tracing a special type of pyrope garnet that is in high in chromium to its source.  The source was found in kimberlite pipes in Siberia.  Several mines were opened causing the Soviet Union to become one of the world’s largest producers of diamonds.

A princess cut diamond mounted in a ring.
Photo by Stephen Durham

After the discoveries in the Soviet Union the scene shifted to Australia where a new type of deposit was discovered in a type of rock called lamproite.  This has resulted in the development of the largest diamond mine on earth, the Argylle Mine.  Some pf the diamonds found in this mine are colored purple an extremely rare type of diamond.

The diamond discoveries in the Soviet Union and Australia led to the discovery of kimberlite deposits in Canada, Brazil and India.  There are also deposits producing diamonds in Arkansas and eastern Colorado and Wyoming

Wednesday, January 19, 2011

What is a Gemstone?

A gemstone or gem is also called by several other names such as precious or semi-precious stones or even jewels. These stones when cut and polished are used to make jewelry or other adornments used for decorations on the human body. Most gemstones are minerals but there are certain rocks such as lapis lazuli that are rocks. Other gemstones are made of organic matter such as amber, jet, coral or pearls even though they are not minerals they are still used in the manufacture of jewelry. Because of this use they are still considered to be gemstones. For the most part gemstones are usually hard, but occasionally some soft materials find their way into jewelry because of certain characteristics that are found attractive. In the past gemstones were also used to create major luxury art forms. The carvings made by the Russian lapidariest Carl Faberge were the last significant works that were made during the 19th century of this type.

A collection of uncut gemstones.
In the West the traditional classification of gemstones goes back to the ancient Greeks. This classification was split between the precious and semi precious stones; this distinction was also made in many cultures around the world.

In our modern world these are the stones that are considered precious diamond, ruby, sapphire and emerald. Some experts include the natural pearl on this list. The list of precious stones is not particularly scientific it rather tells of the rarity of these stones during ancient times. There are many stones that have as high a value as the so-called precious stones that are only considered to be semi precious. An outstanding example of this is the Alexanderite a variety of crysoberyl the changes from red to green tea pending on the type of light it is exposed too.

A faceted blue tourmaline
Most of these gemstones grade from transparent to translucent displaying fine color when they are pure and with no inclusions, and are very hard. The diamond is the hardest of all with a hardness on the Mohs scale of 10. Generally to be an acceptable gemstone asset have a hardness of at least 8 on the Mohs scale. A slight amount of tolerance however is allowed for some gems as soft as 5 on the Mohs scale to be used in jewelry providing it is not exposed to wear such as an approach or pendant.

The words precious or semi precious used in the context of commercial gemstones is often misleading because it implies certain gemstones are intrinsically more valuable than others when it is just not true.