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GARNET
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An iron aluminium silicate which is very hard (7-7.5 on the mohs scale). Due to this it is widely used in jewellery although it is very sensitive to heat which may result in chipping when it is worked on. The more iron a garnet contains the more sensitive to heat it is. Garnets are insoluble in acids, therefore these may be used to remove imperfections on gemstones.
Traditionally garnets are known to be red in colour. These garnets are red almandines and are the most valuable and widely imitated members of the garnet group. They are red due to the presence of iron, which is the most abundant component. Some may contain needle like inclusions of titanium oxide known as rutile which may create a star like optical effect known as asterism.
The word garnet originates from the old French word grenat which means red. However, not all garnets are red. As well as almandine there are 5 other types:- andradite, grossular, pyrope, spressartine and uvraovite.
Garnets can be found in metamorphic rocks e.g. garnet mica schist and granitic pegmatites.
Red almandine garnets can be found in Australia, Brazil. India, Madagascar, Sri Lanka, and Tanzania. The best star garnets showing asterism come from Idaho, USA.
A borosilicate which is green due to the presence of large concentrations of sodium. Although tourmaline is usually black the finest specimens are green. The most valuable gemstones are mid-green because lighter greens are too similar to olivene and darker greens are opaque and do not show a beautiful play of light. Some tourmalines appear different colours depending on the angle from which they are viewed. This is known as pleichroism. Specimens which are pink and green are known as watermelon tourmaline. The colour of tourmaline depends on which element is dominant in it's composition e.g. sodium, calcium, magnesium, aluminium, manganese or lithium.
Tourmaline is usually free of inclusions and is fairly hard (7-7.5 on the mohs scale). It is mainly used for jewellery and may sometimes show alternating lines of lighter and darker shades due to the way light is reflected from it's surfaces. This characteristic is unique to tourmaline and may help to identify it.
It was first imported to Europe from Sri Lanka at beginning of 1700s and is found today in Brazil, Tanzania and Namibia.
