Amethyst - a guide to the February birthstone

Amethyst - a guide to the February birthstone

Amethyst, the birthstone of February, is an enigmatic stone that is found in a range of hues from deep reddish purple to pale lilac. It is a crystalline quartz that sits at a relatively hard 7-8 on the (1-10) Moh's scale and as such it is a fairly easy stone to look after. Higher grade examples can be facetted into a jewellery gem, whereas lower-end material tends to be carved into beads and ornamental pieces. Gemmologists and collectors alike prize amethyst for its deep colour saturation and high clarity. Amethyst derives its purple colouration from a combination of its iron content and the natural irradiation that occurs where the stones are formed, deep below the surface. Amethyst is mainly found in Brazil, Bolivia, Uruguay and Zambia. The Brazilian stones tend to be the largest but they generally do not display the deepest colouration and often exhibit colour banding (in spite of the best efforts of cutters to minimise it). Most collectors prefer stones that originate from Zambia and more recently Uruguay as, although they are often smaller, they tend to display a richer colour. As is the case with most coloured gemstones, hue, saturation and tone are the largest determiners of the value of an amethyst. The combination of even colour, with depth of colour and the absence of visible zoning are the key factors. The most sought after amethyst of all are those that are said to be "Siberian quality," a term that no longer relates to the stone's provenance but rather a red and blue hint within a deep purple combination.

(A rare specimen of Siberian amethyst)

As a general rule buyers prefer darker stones, ideally those with 75-80% tone, combined with secondary blue hues comprising up to 20%, giving greater overall depth to the stone. When secondary red hues appear under incandescent light, this is the hallmark of a truly exceptional specimen. In recent times, lighter stones have actually started to become more popular when set into jewellery pieces. In particular examples with light lavender tones have been successfully marketed as "Rose de France" amethyst. Following on from Siberian amethysts, the general pecking order is graded from AAA to A, where AAA denotes eye clean stones with good colour saturation, AA stones that have small inclusions and/or slightly weaker colour characteristics and A amethyst with significant inclusions and a very light tone. Unfortunately this grading system is not universally recognised or agreed, resulting in some discrepancies from one dealer to another. As eye clean amethyst are so commonplace, most experts recommend that buyers avoid included stones in their entirety and look for amethyst with a high degree of clarity and transparency. This is especially true of lighter stones, in which inclusions are all the more apparent to the naked eye. Due to the abundance of amethyst, poorly cut stones are generally much cheaper so it is important to ensure a symmetrical cut with excellent proportions. The vast majority of amethyst are faceted, as this shows off their dazzling scintillation (or 'sparkle') most effectively, but some cabochon examples can present the stone's colour in a good light.

(Amethyst crystals before they are cut and polished into gemstones)

Amethyst comes in a wide array of carat sizes but very large crystals of fine quality are hard to come by. The largest examples are of Brazilian provenance but they often display colour zoning, which limits their value. The smaller, more intense, amethyst are found in Zambia. Unlike many coloured gemstones, the price per carat of amethyst tends to increase gradually, rather than exponentially, as far as 25 carats, after which it starts to decrease. When quartz crystals contain both purple and yellow colouring (amethyst and citrine) they are referred to as ametrine or 'golden' amethyst. To date ametrine has only been found in Bolivia, where gem quality examples usually display moderate colouration, and some of these have been produced in very large carat sizes. Synthetic amethyst is manufactured in vast quantities and when purchasing stones buyers should inspect the laboratory certificate carefully. It is also important to check if a stone has been treated or enhanced. Heat treatment at 400-500 Celsius can lighten the colour of a very dark amethyst and turn it green (known as prasiolite amethyst), blue, brown, red or yellowy-orange. Irradiation, in addition to heat treatment, can also produce dark brown, orange and yellow colouration. Extreme heat can change a stone's composition completely, turning it into citrine or another quartz variety. Many treated amethyst can experience colour fade if exposed to prolonged sunlight so it is important to consider this practicality when purchasing one.

(Amethyst crystals form from quartz in unique geological conditions)

The value of an amethyst is largely dictated by its colour. In the past, Siberian mines produced the best quality amethyst, featuring a very rich purple with red and blue flashes but the Siberian stocks were exhausted many years ago so the name is now applied to stones that exhibit these desirable characteristics. Amethyst has long been associated with purity and spirituality and it is even said to have the mythical power of saving its owner from the side effects of drunkenness in some folklore! Amethyst is unique among quartz varieties in that it is very rarely found in sizes over 100 carats, whereas many other quartz stones have been produced that are over 1,000 carats. There are however some very exceptionally large amethysts on display in the Smithsonian Institute's gem collections. Some of the other marketing names that have been given to amethyst include the slightly misleading "Madeira topaz," which is actually a heat treated brownish-yellow amethyst. Another example of this branding technique is "Palmyra topaz," which is in fact a lighter shade of amethyst and stones that are heated to a reddish colour are also sold as "Spanish topaz." Erroneously violet sapphire is sometimes referred to as "Oriental amethyst" and Kunzite is sometimes wrongly called "Lithia amethyst." Gemmologists the world over agree that amethyst is a gem species in its own right that is distinct from topaz, sapphire and kunzite so all of these names are misleading and should be avoided. Above all else, it is essential that buyers use a reputable jeweller or gem dealer when sourcing coloured gemstones.

(Despite being found in abundance, very fine specimens of amethyst are hard to come by)

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