A collecting guide to semi-precious gems
Do you know your alexandrites from your garnets, why opals appear rainbow-hued and peridot is known as the ‘evening emerald’? Illustrated with gems offered at Christie’s
Alexandrite was discovered in 1830 in the Ural mountains of Russia, and named after the tsarevich, the future Alexander II. It is a variety of chrysoberyl, but while the chrysoberyl is yellowish-green in colour, the alexandrite changes with the light — appearing greenish during the day and reddish in incandescent light.
Today, it is also found in Africa, Sri Lanka and Brazil, but the most sought-after alexandrites remain those from Russia.
The amethyst is the purple variety of quartz and was once considered a ‘cardinal gem’ — the category of rare and precious gems, traditionally worn by royalty and the clergy, that comprises rubies, sapphires, diamonds and emeralds.
After large deposits were discovered in South America in the 19th century, however, the amethyst was demoted to the semi-precious category. Its name comes from the Greek words for ‘not’ and ‘intoxicate’: it was believed to protect its wearer from drunkenness.
The aquamarine is the light-blue variety of beryl. It is named after the Latin for ‘seawater’ and was believed to protect its wearer at sea. Pure beryl is colourless — when eyeglasses were first created in the 13th century, small plates of beryl were often used because manufactured glass was not clear enough.
It is mined in Pakistan, more than 4,500 meters above sea level in the Trans-Himalayan mountains, and typically yields large crystals, free of inclusions.
The garnet is a group of common gemstones that includes the almandine, pyrope, demantoid, tsavorite and spessartine. It is mined all over the world — from Australia to the United States of America, and from China to India. Colours range from red to blue, yellow to green, but the most popular are orangey-red and green.
Red garnets — also known as carbuncles — were written about by Pliny and worn in the Middle Ages; they have also been found on ancient mummies.
The moonstone is a member of the feldspar family and was once believed to be made of frozen moonlight. It is notable for its whitish-blue sheen, known as adularescence after the Adula mountains in Switzerland, where moonstones of great quality were originally mined.
Today the stones — which are also known as adularia — tend to be mined in Sri Lanka and India, and cut as cabochons to display their colour to best effect.
The opal is one of the oldest gemstones on record. The precious variety is known for its remarkable ‘play-of-colour’, the optical phenomenon that occurs when light enters the stone, is diffracted by its internal structure, and leaves in a flash of spectral colours.
Because of this quality, which allows the beholder to see the colours of all the precious gems at once, precious opals were long associated with good fortune and supernatural powers. Until the mid-20th century, they were mostly mined in Slovakia. Now Australia is the primary source.
Peridot is the gem form of a mineral called olivine, which hints at its vibrant green colour. In Ancient Egypt, it was allegedly mined at night because it was impossible to see during the day. In Ancient Rome it was called the ‘evening emerald’, because its colours did not darken at night.
Peridots and emeralds have often been confused, famously in Cologne cathedral, where the large green peridots adorning the Shrine of the Three Kings were long thought to be emeralds. Today, peridots are mined in Burma, Arizona and China.
The spinel is one of the most ancient known gemstones and comes in a variety of colours, from yellow to green, blue, red, pink and transparent. The most valuable colour is red, and in ancient times, red spinels were often confused with rubies.
The confusion came from a misunderstanding between the Western world, which considered the most valuable red gemstone to be the ruby, and the Eastern world, for which it was the red spinel. Which explains why the Black Prince’s Ruby set in the Imperial State Crown of England is actually a 170-carat red spinel.
As the name indicates, tanzanite comes from Tanzania in Africa, where it was first discovered in the late 1960s. It is actually a blue variety of zoisite, but was renamed shortly after its discovery by Tiffany & Co, which went on to launch a major marketing campaign, claiming it could only be found in two places: in Tanzania and at Tiffany’s.
Tanzanite is one of the most recent gemstones to have known such commercial success: it has a vivid bluish-purple colour, is often free of inclusions, and it yields large cut stones.
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Tourmaline can be found in a range of colours and even — as in the case of the watermelon tourmaline, which is green, red and sometimes yellow — bear several colours at once. It is a relatively young gemstone in the respect that it was only recognised as a mineral in its own right in the 1800s.
Before that, green tourmalines were thought to be emeralds, red tourmalines rubies, and so on. In the late-1980s, a new variety of electric-blue tourmaline was discovered in Brazil. Named the Paraiba tourmaline, it is mined in relatively small crystals, quite rare and, as such, more valuable than other varieties.