Azurite Hydrous Copper Carbonate a Blue Gem.

Botrydoil Azurite from the Apex Mine of Utah.
Photo by Rob Lavinsky

Azurite is one of the lesser known copper ores that is produced by the weathering of copper deposits. The mineral is also known as chessylite named after the Chessy-les-Mines near the city of Lyon in France. This mineral has been known to the ancients and is mentioned in Pliny the Elder’s, Natural History.  Because of its deep blue color has also been used as a pigment that is a deep blue color. Azurite is usually found in the desert where the low humidity causes its cousin malachite to lose water.  This is where it is often found in the company of its cousin malachite.  Both minerals are essentially the same copper carbonate except azurite contains more water in its crystals.  Over time this water will evaporate causing the azurite to change to malachite.

Typically azurite crystals are monoclinic that when they are large enough to be seen will appear is dark blue prismatic crystals. In nature azurite crystals are massive to nodular with some of them forming stalactitic masses. Any specimen of azurite over time will lighten in color as its surface retrogrades in the malachite. The mineral is soft with a Mohs hardness of 3.5 to 4. It displays a specific gravity ranging from 3.77 to 3.89. The mineral azurite is destroyed by the application of heat losing both carbon dioxide and water forming a type of black powdery copper oxide. A drop of acid placed on the specimen will cause it to effervesce like any other carbonate.

Azurite and malachite
Photo by Rob Lavinsky

Some of the earliest uses of azurite are a blue pigment in artist’s paints. In this use it was finely ground, and then mulled with linseed oil and turpentine. The mineral is capable of producing many different shades of blue depending upon the fineness to which it is ground. When it is mixed with oil, as in oil paints it turns slightly green. On the other hand if it is mixed with egg yolk it turns a gray green. In older paintings the azurite has turned into malachite displaying a greenish color. Many times what was azurite was mislabeled as lapis lazuli that in times past was a name that was applied to many other blue pigments. True lapis lazuli that was used as another blue pigment came primarily from Afghanistan during the Middle Ages. Azurite was a common mineral found in Europe in the same timeframe.

Lapis Lazuli, notice the difference in color from azurite.

There were sizable deposits of azurite that were mined near Lyons, France during the Middle Ages. Azurite was also mined during the 12th century in Saxony and the various silver mines that were located there.

If azurite is heated it turns black whereas the more expensive natural ultramarine remains blue when it is heated. If the mineral is gently heated it produces a deep blue pigment that is often used by the Japanese in some of their painting techniques.

Azurute surrounded by malachite.
Photo by Rob Lavinsky

Occasionally azurite finds uses beads and other types of jewelry; it is also used as a decorative stone. The thing that keeps it from being used more commonly as a gemstone is its softness and the fact that it weathers easily reverting back to malachite.

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Amazonite the Green Feldspar Gem

Amazonite a green variety of microcline feldspar.
Photo by Rob Lavinsky

Amazonite that is also cold Amazon stone is a green colored variety of microcline a type of feldspar. It got its name from the Amazon River when certain green stones were formerly obtained from the land surrounding the river although it is doubtful that any of this green feldspar occurs anywhere in the Amazon region.

Amazonite with terminated smoky quartz crystals.
Photo by Rob Lavinsky

Although amazonite is a mineral having a limited occurrence it was originally obtained from the Miass district in the Ilman Mountains about 50 miles southwest of Chelyabinsk, Russia. It was found here in granitic rocks in small quantities. It can also be found in the United States as high-quality crystals found in the area of Pike's Peak, Colorado. Some spectacular specimens displaying amazonite along with smoky quartz crystals in a matrix of albite and orthoclase feldspar have been found around Pike’s Peak. Another area where amazonite is found in Colorado is at Crystal Peak in El Paso County. There are other places in the United States were amazonite has been found associated with pegmatites like it is in Colorado.

Amazonite cut in cabochon.
Photo by Eurico Zimbres

For many years many people assumed that the bright green color of amazonite was because of copper compounds that are often similarly colored having bright blue or green colors. Recent studies that have been undertaken suggest that the blue-green colors of amazonite are actually caused by a small amount of lead and water as crystal defects in the feldspar.

A lapis lazuli pectoral made in Ancient Egypt.  A small piece of amazonite can be seen as the round stone at the bottom of the pectoral.
Photo by Henryk Jan Dominiak 

Amazonite has been used for years when it has been cut and polished as a gemstone, but as gem it is not advisable to use this in the rings because it is brittle and fractures is easily.  However, when cut en cabochon is readily finds use in pendants and lockets.  Large masses of this stone are used for carving.

Gem Bearing Cavities in Pegmatites

Cavities in rock.
Photo by Ustill

Just about any kind of rock can have cavities some pegmatites are particularly prone to this feature, and it is in these cavities that most gemstones are found. There are many causes of these cavities, but one of the most common is a gas bubble. In many pegmatites cavities can also be formed by a lack of sufficient magma to completely fill the space the pegmatite is occupying.

Cavities are found in pegmatites generally come from one source that is the lack of sufficient material to fill the pegmatites solid. Some of these cavities are quite small and are measured in millimeters. Larger ones are measured in centimeters, and still larger ones are measured in meters. The latter are the most uncommon form of cavity. When they are found in the process of gem mining they often become legendary containing several million dollars worth of gemstones.

Rubbilite tourmaline crystals found in a gem bearing cavity in pegmatite.
Photo by Rob Lavinsky

A cavity that has not been affected by Earth movements grows a fine crop of gem crystals that are attached to the matrix. These crystals are often covered with a fine layer of clay that forms from the last of the mineral bearing solution in the cavity. One might say from the leftovers. Less than one in ten of all the cavities that are found are of this variety. Originally these pegmatites were under considerable pressure as they were formed several miles beneath the surface of the earth. As they were brought up to the surface of the earth by the process of erosion they lost this initial pressurization through the process of a diffusion of their internal gases through the pores in the rock.

Beryl, Cassiterite and Muscovite from a pegmatite cavity.
Photo by Rob Lavinsky

It's the same cavity is fractured by Earth movements so that it loses its internal pressurization crystals that it contains are shattered, and the fragments fall to the bottom of the cavity. Another feature of this type of cavity that is now open for the atmosphere allowing it to be filled with clay that accumulates in the cavity from sources found on the surface of the earth.

Finally, some cavities that remains sealed from the atmosphere are visited by a later type of mineral bearing water that causes the gem crystals inside him to be etched away partially or wholly.

These cavities are known by many names including cavity, geode, pocket and vug.