The Story of Gems: Amblygonite
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The Story of Gems: Amblygonite

This is the story of the rare gem; amblygonite. This is a rare stone found mainly in Brazil where it is mined as lithium ore.

Although amblygonite is an ore of the metal lithium that is found in lithium and phosphate rich pegmatites it also finds use as a faceted gemstone. Its composition is (Li, Na)AlPO4(F, OH), Lithium Sodium Aluminum Phosphate Fluoride Hydroxide. This is a mineral that has worldwide occurrences, but mostly amblygonite gemstones have their main occurrences in Brazil where it is mined for its lithium content. At times this mineral can make up a significant percentage of the pegmatites that it is found in although it many times this fact is often overlooked. The mineral is easily confused for other host rock minerals that is found with such as quartz and albite. Although it has a general appearance very similar to these other minerals by the use of a flame test that causes amblygonite to color the flame red. Another way that it can be differentiated is because it has greater density as well as an unusual cleavage.

The name amblygonite is the derived from the Greek words meaning “blunt angles” that refer to it variable angled cleavages. This mineral is blessed with at least four different cleavages that occur at different angles from each other having different qualities for each cleavage. In this it is quite different from either quartz that has no cleavage or albite that only has to cleavages that are close to 90° apart.

Because the stone is so compact and has a higher specific gravity of around 3.05 making it much higher than either quartz or albite. But if you look at amblygonite's formula you can quickly see that it contains the elements sodium, lithium, aluminum, hydrogen, fluorine, oxygen and phosphorous. This would leave you to expect this stone to have a much lower specific gravitie is then three whereas both quartz of albite have lower specific gravities. The reason for this is because the individual atoms in the mineral are more closely packed to gather the day are in the other two. This is the cause of the higher specific gravity of amblygonite.

The mineral amblygonite often forms a solid solution series with a mineral that is often associated with called, “montebrasite.” There are no visible features between these two minerals to differentiate them causing many specimens that are labeled amblygonite are in truth montebrasite.

As a gem amblygonite is most often mined in Brazil but there are other places in the world where you can find the stone the most notable of these regions is in the former country of Burma. The stones have an attractive yellow color with a retraction index of 1.61 and 1.64. This is a relatively low retraction index but it is higher than Burrell or quartz putting it on a par with topaz and tourmaline. There is a whole family of minerals that are called the amblygonite group with other alkali metals substituting for the lithium normally found in amblygonite. All of these minerals share one thing in common they can all be gemstones.

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