Opals are unique gemstones. Although geologists don’t really have a conclusive answer as to exactly how these stones form, one thing is for certain – water is involved. They believe that water carries silica as it seeps into the cracks of rocks. The silica solidifies and then traps water, which is what causes the phenomenon known as “play-of-color”. The diffraction from the water causes the colors in the gemstone to change while tilted under light.
Water content in opal ranges from 3 - 21%. Opals aren’t very hard stones as they range from 5 ½ to 6 on the Moh’s hardness scale, so they can be easily scratched. Thus, opals are somewhat fragile and need to be worn with care. A somewhat unexpected thing that can also happen to an opal is something called “crazing”, where tiny cracks form in the gem. This occurs when the opal becomes too dry. The stone in the image above is exhibiting “crazing”. This property increases the care that this type of gem may need.
Lightning Ridge is known to produce the most spectacular black and gray opals. These opals are rare. No other known locality in the world has natural, black opals like Lightning Ridge. Be certain you are looking at a genuine black opal as opposed to a treated Ethiopian opal. They are prevalent in the market place, and often misrepresented.
