This week’s What’s on the van? comes from Monica Price, Head of the Museum’s Earth Collections.
The fact that there are two different examples of precious opal shown on the van might just give you a clue to the fact that opal is one of my favourite minerals! The piece shown here is nearly 9cm across. It has a lovely play of colour and comes from Australia. When the Museum reopens next year, you will be able to see it in our gemstone display.
Precious opal, with its spectral flashes of colour, comes from Australia, Slovakia, Honduras, USA, and just a few other countries of the world. But there are lots of other kinds of opal. Common opal looks rather dull by comparison, and is a hard, brittle, waxy-looking material. As its name suggests, it is found in many different places.
Sometimes an impurity can give opal a lovely colour, for example a tiny bit of nickel or copper turns it a pretty shade of green. Opal can fill the pores in fossil trees to form ‘petrified wood’ or ‘tree opal’, and the banding and graining of the original wood may be beautifully preserved.
‘Hyalite’ is a kind of opal found in volcanic rocks. It is perfectly transparent, looking just like bubbly colourless glass. ‘Menilite’, which comes from Ménilmontant in Paris, is sometimes described as ‘liver opal’ and that gives a rather good clue to its colour and appearance!
Opal never forms crystals, and instead it consists of tiny spherules of silica (SiO2). The spherules in precious opal are of particular sizes and are very neatly stacked. When white light strikes them, they diffract the light to give a rainbow of colours. No wonder precious opal makes a lovely gemstone!