1. Opal is composed of millions of sub-microscopic spheres of silica packed together (imagine a box filled with ping pong balls!). This structure leaves tiny gaps that bend and split rays of light that hit the opal’s surface and is what causes the beautiful play-of-colour that opal is famous for.  These spheres range in size from about 0.1 to 0.2 microns (0.1 microns is one ten-millionth of a metre) and different sizes create different colours, so when you look at an opal and see patches of different colours you are looking at areas composed of slightly different size spheres.

2. Opal forms after heavy rains hit very dry ground. The rainwater, which contains amounts of silica, seeps into cracks in the dry earth, and when the water evaporates it leaves the silica behind.  This is why opal is often found in seams and small spaces within the host rock, and also why opal contains up to 20% water in its composition.

3. Opal’s method of formation sometimes yields fascinating results. Occasionally the silica can replace organic matter and produce fossils made from opal, capturing the remains of snails, seashells or plants in this beautiful material. 

4. Some varieties of opal showcase the stone’s formation in cracks and seams within the polished stone. Boulder opal maintains the host rock as part of the stone, often making a feature out of natural shapes and contrasts between the rock and the colourful opal.  Matrix opal displays an array of tiny pieces of colourful opal scattered across the dark surface of the host rock.

5. Although precious opal (varieties that feature the unique play-of-colour that shimmers just below the surface of the stone) is reasonably rare, opal is in general much more common. Material lacking play-of-colour is aptly known as common opal and is found in many places around the world.  Although most is not used for gemstones, some areas produce pretty colours that are used in jewellery.  Peru is known for pastel shades of blue, pink and green common opal.

6. Mexico is famous for a variety of precious opal known as fire opal. This is usually transparent to translucent and bright red, orange or yellow in colour.  It rarely shows play-of-colour but is valued for its vivid hues.

7. Some patterns of play-of-colour in opal are given names that reflect their particular appearance. Harlequin opals have larger rectangular or diamond shaped blocks of different colours, while pinpoint or pinfire stones feature multiple small points of colour.  The kind of appearance you prefer in an opal is simply down to personal choice.

8. Historically opal was found primarily in central Europe and was a rare material prized by the wealthy. The discovery of extensive opal fields in Australia in the late nineteenth century changed this, making opal more commercially available and increasing the variety of colours and patterns.  Some opal-producing areas have become famous for particularl varieties, such as Coober Pedy for white opal (opal with a solid white background to its play-of-colour) and Lightning Ridge for spectacular and highly prized black opal (stones with a dark background and vivid play-of-colour).  Australia is still the biggest producer of opal, but over the last couple of decades Ethiopia has become a significant source and is known for its beautiful crystal opal which is transparent to translucent with little to no body colour and shimmering patches of play-of-colour.

9. The Romans treasured opal as the most precious of all gemstones, believing the ability of this one stone to display the colours of all the other precious gems marked it out as unique and special. Its name, ‘opalus’ means precious stone and it was said to combine the virtues associated with the other precious stones and be the luckiest of them all.

10. More recently opal has been unfortunate to attract a reputation for bad luck. This probably stems back to Sir Walter Scott’s novel Anne of Geierstein, which featured a character who died shortly after her opal mysteriously lost its colour.  Although of relatively recent date (most superstitions are centuries older), this association has really taken hold.  To those of us who are not superstitious the most likely explanation is that opals are delicate stones and very easily damaged, so a stone being lost from a setting or having its appearance altered is relatively likely, and this could well, completely by chance, coincide with another unlucky event! 

And…

11. I couldn’t finish this list without mentioning the discovery of opal on Mars! Studies of the Martian landscape indicate the presence of hydrated silica deposits – the material we know on Earth as opal.  This discovery was one of the indications that led scientists to believe that there is or was water on the red planet, one of the most important scientific discoveries of recent years.