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.

The Different Types of Pegmatites

Pegmatite in mica schist.
Photo by Arlette

If we seem to be staying on the subject of pegmatites it is because they are very important in the production of gemstones. There are over 100 different types of gems that are produced from pegmatites. Most of these pegmatites are chemically the same as granite, and the term actually refers to their texture rather than their composition. There are three recognized varieties of granitic pegmatite, and one that is recognized in gabbro. All pegmatites containing crystals that are larger than 2 cm in diameter and can reach truly gigantic proportions. There is a feldspar mine in the Ural Mountains of Russia that has been producing feldspar from any single crystal for many years.

Pegmatites were classified in the mid-20th century by Buddington with a depth zone classification of granitic rocks in 1959. Cerny’s revision of 1991 is the most commonly when used today for the classification of pegmatites. Basically these classifications are based on the depth at which pegmatites were emplaced, metamorphic grade as well as minor element content. Under this classification Abyssal pegmatites are the highest grade that have been in place that either high or low pressure. Muscovite grade pegmatites were emplaced in high-pressure, but a lower temperature. Rare element pegmatites were emplaced at even lower pressures and temperature. In this classification the miarolitic pegmatites having large cavities were emplaced at shallow depths. The most gemstones are found in the miarolitic pegmatites within the enclosed cavities.

Beryl with microcline found in a pegmatite cavity.
Photo by Rob Lavinsky

The simple pegmatite is composed of feldspar, quartz and lesser amounts of muscovite mica. This is the simplest type of pegmatite that is found that can range from tiny veins a few millimeters wide to great masses of pegmatite that can cover several acres, and be meters wide. For the most part simple pegmatites have a very uniform composition, and little variance in the size of the crystals from one side to the other. Approximately 80% of all pegmatites found in nature are of this type. There are very few gemstones found in the simple pegmatites. Most of them contain white colored feldspar and milky quartz with sometimes bands of muscovite mica parallel to the sides of the vein. Under the above classification this type of pegmatite would fall under the classification of being an abyssal pegmatite.

Red tourmaline on lepidolite and quartz from a pegmatite cavity.
Photo by Rob Lavinsky

The rare element class of pegmatites is further divided into the sub classifications LCT and NYF. LCT pegmatites contain lithium, cesium and tantalum. NYf stands for a niobium, yttrium and fluorine. These are perhaps the rarest types of pegmatite. Most of the gemstones bearing pegmatites are of the LCT variety. The closer they form to the surface the more apt they are to have miarolitic cavities containing gemstones.