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Amethyst – Birthstone for February

Amethyst, is part of the quartz family of semi-precious gemstones, in its purple iteration. Quartz is one of the most abundant minerals and it has been estimated that quartz constitutes about 12% of the earth’s crust. Its chemical composition is SiO2 (silicon dioxide).

Colour Importance

Purple has been considered Regal since ancient times, as purple dye was first made by the boiling of Murex shells, found in the Mediterranean Sea, near the city of Tyre, in present day Libya. A very large number of shells had to be boiled to extract the purple dye, it was extremely malodorous work carried out by slaves. Because of the limited supply of dye, it was reserved only for high status people, and in fact in the paintboxes of today one can still find a colour called Tyrrean Purple, which is a very intense purple colour.

The finding of natural purple stones, or Amethyst crystals, only made the colour more desirable, and since Antiquity Amethyst has been continually utilised by various groups to signify power and status. Due to the abundance of Amethyst it is now very fairly priced, making it affordable for most people. In Ancient times, it was scarce, and considered one of the Cardinal Gemstones, together with Sapphire, Diamond, Ruby and Emerald,  precious above all other gem stones. It was only after the discovery of large deposits of Amethyst in Brazil, that the price fell very significantly. Natural Brazilian Amethyst is a light purple with no visible colour zoning.

The actual cause of colour in amethyst is still uncertain, but the two theories given most credibility are

(a) a concentration of molecules along the contact zones of twinned lamellae or

(b) disturbance of internal molecular structure by irradiation from adjacent rock masses.

  • Amethyst if left out in the sun will eventually lose its notable purple colour.
  • Most commercially available citrine is obtained by heat treating amethyst.
  • In 1953, a transparent green gemstone was obtained by heat treating amethyst mined from Montezuma, Brazil. It is called Prasiolite. The use of the term Green Amethyst is a misnomer, Amethyst, by definition, is a purple variety of quartz.

 

History

Amethyst was originally thought to prevent drunkenness, and the name comes from the Ancient Greek, “A” meaning not, and “Methisko”, or Metho in Modern Greek , meaning intoxication. The Ancient Greeks wore Amethyst amulets and drunk from carved Amethyst drinking vessels in the belief that it would prevent intoxication.

The Ancient Egyptians used Intaglio Carved Amethyst Crystal Gems for use as Official seals on Papyrus documents, and Clay tablets, or as items of adornment.

Medieval European Soldiers wore Amethyst amulets as protection in battle, and to keep them cool-headed.

Beads of Amethyst have been found in Anglo-Saxon graves in England. In the Anglican Church, Bishops often wear an Episcopal Ring set with an Amethyst, an allusion to the description of the Apostles as “not drunk” at Pentecost in Acts 2:15.

Once again, in the Middle Ages Amethyst was considered a symbol of Royalty, and used to decorate English Regalia.

The highest-grade Amethyst (called Deep Russian, or Deep Siberian) is of a very deep colour with flashes of red or blue evident, similar to the deposits found it that country. As the colour of Amethyst  is not usually distributed evenly throughout the crystal form, it takes skilful gem cutting  in order to have a consistent colour visible on the front facet of the gem.

 

Natural Vs Synthetic

Quartz is unusual among minerals in that it is normally very pure, nearly 100% SiO2 (silicon dioxide), and has very constant physical properties. Quartz is also unique in having a spirally arranged growth structure and has a hardness of 7 on the Moh’s Scale

Amethyst occurs in primary hues from a light lavender, through a pale violet colour, through to deep Purple. The colour in amethyst is often zoned or banded due to complex twinning. Angular or straight zoning with colourless or violetish blue zones next to purple areas characterizes natural amethyst. The presence of zones of only light and dark purple or the complete absence of zoning indicates synthetic origin, Inclusions can also help with identification “Zebra Stripes” are considered as a diagnostic inclusion in amethyst but are rarely seen in cut gem quality stones.

Natural amethyst almost always shows polysynthetic twinning on the Brazil law.

Quartz is also a uniaxial mineral and will show an Interference pattern like a bulls eye where the isogyres (“brushes”) create a cross pattern with the centre bulleyes being clear or hollow. This is due to the spiraling growth nature of the quartz crystal structure. 

This can be seen by looking at the stone thru crossed polarised lens (polariscope) and placing a conoscope over the optical axis of the stone. You can try this at home with 2 pairs of polarised sunglasses, turn the sunglasses at 90 degrees to each other so they are “crossed” then put the stone between the 2 glasses, with a, light source form the bottom, then use a clear glass marble  placed close to the stone and keep rotating the stone until you see something like this image, while looking down on the tone from above the 2 pairs of glasses. Both natural and synthetic amethyst will show this figure.

The trick in spotting the synthetic is in the twinning. The fact that almost all natural amethyst is twinned according to the Brazil Law. A natural amethyst will show twining by a rainbow of colours in a spiraling direction where synthetic amethyst the rainbow of colours will be perpendicular to each other.

Synthetic Amethyst has been produced since the second world war, in a single crystal form, which is ideal for use in watches and transistors that require quartz movements to ensure accuracy.

February Gemstone

The allocation of Gemstones to the various months of the year, or signs of the zodiac, is a relatively recent phenomenon. Although it is recent, one can see that it follows a very long tradition of assigning various qualities to different stones, and over thousands of  years Amethyst has been highly prized.

For those with February birthdays, an item of Amethyst jewellery is a considerate and thoughtful gift, full of rich symbolism, and if it really does prevent intoxication, so much the better!

Written by Nicola Bessell-Browne and Rowena Mitchell

References

https://www.gia.edu/doc/A-Simple-Procedure-to-Separate-Natural-from-Synthetic-Amethyst-on-the-Basis-of-Twinning.pdf

GAA Course Notes Chapter 15

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