Opal is a form of hydrated amorphous silica which may contain water with the range of 3 to 21 % by weight but is usually in the range of 6 to 10%. It is the internal structure of the silica present within opal which causes the diffraction that causes the play of colour that we love! The silica spheres need to be packed down into a closely packed lattice for the play of colour to be present. The differing colours that are observed are then determined by the spacing between the planes and the orientation of planes on which the silica is packed.
Up to 97% of the world’s precious opal is sourced from Australian mines namely Coober Pedy, Mintabie, Andamooka and Lightning Ridge. The very best black opal being mined is in the opal fields surrounding Lightning Ridge in New South Wales. Opal fire or play of colour from this area can come in all hues of the light spectrum with a red and blue on black opal potch being a highly sought after and expensive colour combination. The other opal sources in Australia also produce very high quality opal of all colours however Lightning Ridge is the home of Black opal. Opal has been affectionately named the mother of all gemstones for the myriad of colour play that is possible within the gemstone. Opal can show a myriad of patterns within its play of colour with some well known patterns being harlequin, chinese writing, mackerel and Broadflash. These are just a few of the possible patterns found in opal.
It is amazing that by just moving an opal just a couple of degrees the stone can change drastically where one would think they were looking at a completely different gem. That is the beauty of opal; they are very unique and they change characteristics depending on the lighting and the angle that they are viewed.