Crystal system:                 Orthorhombic                                   Hardness:            8,5
Density:                               3,5 – 3,84                                             Streak:                  White
Colour:                                 Various shades of green, yellow, brownish to green black (can be raspberry-red under incandescent light when chromium is present)
Cleavage:                            Distinct, imperfect
Occurrence:                       Found exclusively in pegmatites and in pegmatite eluvium
Habitat:                                Crystals are tabular or short prismatic, prominently striated.
Composition:                     Oxide mineral – Spinel group      BeAl2O4

Chrysoberyl was discovered in 1789 and was described and named by Abraham Gottlob Werner in 1790. Werner worked at the Freiberg Mining Academy from 1790 to 1793 and was known as one of the most outstanding geologists of his time.

The mineral or gemstone chrysoberyl is an oxide of beryllium and aluminium. The name chrysoberyl is derived from the Greek words “chrysos” meaning golden and “beryllos”, which was used at the time to describe water-blue gemstones. Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Chrysoberyl is the third hardest frequently encountered natural gemstone and is listed between corundum (9) and topaz (8) on the Mohs’s scale.

Notable occurrences include Brazil, Sri Lanka, Myanmar, Madagascar, Zimbabwe, Tanzania, Russia and Australia.

An interesting feature of chrysoberyl crystals are that cyclic twins are called trillings (also referred to as sixlings). These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each “twin” orientated at 120o to its neighbour. If only two of the three possible twin orientations are present, a “V”-shaped twin results.

There are three main varieties of chrysoberyl, namely ordinary yellow-to-green chrysoberyl, alexandrite and cat’s eye or cymophane.

The famous light green, transparent Hope chrysoberyl, housed in London, weighs 45 carats. Mention is also made of a large chrysoberyl crystal that weighed an estimated 225 000 carats (45 kg). It was unfortunately destroyed during the blasting of a pegmatite near Paris Maine.

Alexandrite’s most distinctive property is that it is dichroic, i.e. it changes its colour from either green or red depending on whether it is viewed in daylight or artificial light. Gem-quality alexandrite demands high prices in the gem trade and crazy prices of up to $100 000 have been paid for faceted stones which weigh more than one carat. According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld and named alexandrite in honour of the future Tsar Alexander II of Russia. Nordenskiöld’s 1830 initial discovery occurred as a result of an examination of a newly found mineral sample he had received from the Ural Mountains that he identified as emerald at first. The first emerald mine had been opened in 1831.

The alexandrites from the Ural Mountains are considered to be the best in the world. Between 1840 and 1900, Russia was the primary source of alexandrite. During this period the Takovaya district on the eastern flank of the central Urals was mined extensively and many large emeralds and alexandrites were found. No significant amount of new Uralian materials have, however, been reported since the Russian revolution in 1917.

To date, the largest alexandrite crystal specimen found in the Urals measures an astonishing 25 cm x 14 cm x 11 cm and weighs 5,724 kilograms. One of the largest faceted alexandrites weighs 66 carats and is now part of the Smithsonian collection.

The largest uncut alexandrite of gem quality, however, was discovered as recently as 1967 by the founder and chairman of Amsterdam Jewellers, Jules Roger Sauer, in the Jaqueto district, Bahia (Brazil). The stone is named the Sauer Alexandrite and weighs….wait for it…122 400 carats and is held in Sauer’s private collection at Amsterdam Sauer in St. Thomas, Virgin Islands. That is a whopping 24,5 kilograms and it measures 28,5 cm x 28 cm x12 cm. Unfortunately I could not find any photos of this monster.

Cat’s eye or cymophane exhibits pleasing opalescence that reminds one of the eye of a cat. When cut to produce a cabochon, the mineral forms a light-green to yellow specimen with a silky band of light extending across the surface of the stone.

Cat’s eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found. The colour in yellow chrysoberyl is due to Fe3+ impurities.

Cat’s eye really became popular at the end of the 19th century when the Duke of Cannaught gave a ring with a cat’s eye as an engagement token. This was sufficient to make the stone more popular and increase its value greatly. Until that time, cat’s eye had predominantly been present in mineral collections. The increased demand in turn created an intensified search for it in Sri Lanka. Early 20th century prices did go up as high as $8 000 for a cut stone, which was very expensive for those days. The largest cut cat’s eye is 465 carats and is referred to as the “Eye of the Lion” and is from the Sri Lanka deposits.

Locally there are unfortunately a limited occurrence in both the quantity and quality of specimens. I guess we cannot have the best of the species for all the minerals. Chrysoberyl comes from Northern Cape pegmatites, green to yellow crystals occur at Daberas, Middel Pos, Wolfkop and Leeuwkop.  In Limpopo it has also been found in the Letaba district and near Piet Retief in Mpumalanga. In Namibia chrysoberyl is found at the Neu Schwaben pegmatite and rarely, in pegmatites in the Tantalite Valley in the south of the country. In Zimbabwe superb specimens of alexandrite have been found at the world famous Novello claims close to Masvingo..-JDJ

References:
Cairncross, Bruce, 2004 – Field Guide to Rocks & Minerals of Southern Africa.