Topaz has the chemical formula Al2SiO4(F,OH)2, a silicate mineral of aluminium, fluorine and hydroxyl. A hardness of 8 on the Mohs scale, it is about the same as emerald and below that of ruby, sapphire and diamond.
This stone can be found in flawless crystals of massive size that have been faceted into gemstones weighing several kilograms. The largest cut gemstone in the world is the ‘El-Dorado Topaz’ which weighs a massive 6.2kg (31,000 carats) from an uncut 37kg stone. It is an emerald cut yellowish/brown colour with good clarity and is on display in Madrid, Spain.
Without impurities present the stone is clear, but most gems are not. Colours ranging almost the whole spectrum are known and the reason why it can be mistaken for other semi-precious stones. Topaz is differentiated from other gemstones by observing and comparing the hardness, crystal structure, density and fluorescence. Topaz may also exhibit a property called ‘pleochroism’ where more than one colour can appear in the stone which changes with the angle of view.
The colour of these stones is frequently changed by using radiation and then heat to intensify pale colours. Deep blue topaz in nature is quite rare, so white (colourless), grey or pale blue or yellow stones are treated to produce a deeper more desirable shade of blue. The colour change is permanent unless the stone is retreated. Other colours of topaz (treated using irradiation without a secondary heat treatment) may fade with prolonged exposure to sun light. Orange topaz is the birthstone for November, reds, violets and intense pinks are the least common and command the highest prices.
Most blue gemstones start out as clear or slightly tinted and are subject to the radiation that turns them sky blue (electron bombardment) or a slightly green or grayish ‘London Blue’ when neutron bombardment is used. To produce a stone with very highly saturated colour known as ‘Electric Blue’ or ‘Swiss Blue’, both forms of irradiation are employed. Where neutron bombardment has been implemented a very small amount of residual radioactivity occurs. These gemstones are held for up to a year before being released to gem cutters and the public for safety reasons.
Blue stones are the most common and can be found cut into almost any shape. Although topaz is a ‘hard’ stone, it has ‘perfect cleavage’ (as do diamonds), which means if hit hard it will fracture easily.
The vivid colour and high lustre of a blue topaz makes it a very popular gem, 2nd behind sapphire (according to ‘Colored Stone’ a jewellery magazine) in sales. Unlike diamonds which are graded under 10x magnification, topaz is graded by eye in the same way as emeralds. A stone without visible inclusions is said to be ‘eye clean’ or flawless and are widely found.
The factor that most affects the value of these gemstones is the hue and saturation of the colour. Yellow and orange stones also increase in value where more intense pink or red secondary hues are present. Size becomes a factor in the price (per carat) for gems over 5 carats and again jumps for stones over 10 carats.
As this gem is widely found in large quantities of good quality, it is seldom synthetically created although it can and has been done.