Information from the Jewellery’s Directory of Gemstones
Within this species lies a gemstone of drama and intrigue that is considered, along with the sapphire, to be the finest blue stone in existence. Tanzanite, despite its scarcity, has not only gained a remarkable public awareness in a very short space of time, but has become an object of desire worldwide.
Tanzanite is a rare and beautiful transparent indigo-violet gemstone that is found in just one area of Tanzania - and nowhere else. It is by far the most important stone of the zoisite species but a relative newcomer to the gemstone industry, having first been recorded only in 1967. Tanzanite was recognised as a major gemstone when it was launched on the international marketplace, to great success, by the New York jewellers Tiffany & Co. in the late 1960s. In an industry where colour has become a critical factor for jewellery buyers and designers alike, tanzanite plays an important commercial role. For many people, a large blue sapphire would be too expensive to contemplate, yet a large blue tanzanite provides a colourful alternative that is much easier on the pocket.
The colour ranges from deep sapphire blue to violet-blue to soft lavenders and lilacs. Faceted tanzanites often have a violet-red or greenish yellow flashes due to pleochroism, in which different colours and intensities of colour can be seen when the material is viewed from different angles. The stone is traditionally heat treated to produce a more attractive and intense blue colour and to lose and greenish tones. Tanzanite can exhibit a slight colour change: in daylight, good-quality material will take on an ultramarine to sapphire-blue colours; in artificial, incandescent light it will appear to be more of an amethyst-violet colour.
Common inclusions in tanzanite are tiny crystals in various colours that look like spots, as well as clouds of miniature particles, fine hairlike needles and larger rods of different crystals. The hairlike inclusions give an unwanted hazy appearance to the stone and can cause damage during heating, so faceted tanzanites should ideally be ‘clean.’
Cut is an important factor. The material does not have the brilliance of a sapphire and so faceted trillion, cushion and brilliants cuts make the stone appear more showy and bring out the hot pink highlights. Baguette and emerald cuts (step-cuts) require material that has a very good colour and clarity, so that the stone does not look pale or dull.
Green or chrome tanzanite has been discovered, the colour ranging from yellowish green to grey-green to bluish green. The availability of this material is poor; it is not commercially viable.
Thulite is a dense, opaque pink to red variety of zoisite that is coloured by manganese. It is normally cut as cabochons or beads or used for carvings. it has a low commercial value, and may be confused with rhodonite.
Discovered in Tanzania in 1954, this opaque, green zoisite rock contains black hornblende inclusions and large, low-grade opaque rubies (called ruby-in-zoisite). It has an attractive colour contrast that works well in carvings, beads and cabochons. Despite containing rubies, it is only of moderate value.