| Unlike other gemstones natural opal does not have a crystal structure, and the texture consists of amorphous silicon dioxide having submicroscopic silica spheres. Depending on the size and how these silica spheres are arranged together determines how they diffract light. These silica spheres have diameters that range from 0.15 to 0.30 microns (one thousandth of a millimeter ) which is a little smaller than the wavelength of visible light which ranges from 0.4 to 0.7 microns in diameter. In order for opal to be considered precious opal the silica spheres must be uniform in diameter and arranged in neat rows. A precious opal that has flashes of red will also contain all the other colors of the spectrum as the opal is seen by all other angles. It is interesting to know that this structural color of opal is produced by light, conveyed by the eye and is then understood by our minds.
To this day I am fascinated on how challenging Natural Opal is to cut and polish. Since I know that Opal with flashes of red and yellow are highly prized, I do my best on keeping as much of these colors as I possibly can into an opal cabochon or incorporating it into my inlay designs. Generally, if I am inlaying natural opal I will design the jewelry around the stone so there is minimal cutting of the opal. I never know what the stone will look like until it is finished. As I get more experience grading opal, all other types of stones that I work with become naturally easier to work with. I realize that all types of natural opal are magnificent gemstones and display unique characteristics. But out of all the natural opal in the world there is only a couple types of opal that I am comfortable in using.
The most prized natural opals that I use for my inlay jewelry is natural opal from Australia and Brazil. Both Australian and Brazilian Opal have a very good reputation for producing some of the most stable and precious natural opals in the world. These opals are similar in both their flashing colors and the intensity of the play of color. All natural opal has a Mohs hardness that ranges from 5.0 to 6.5 which are considered to be softer stones. Suprisingly, Brazilian Opal is hard to distinguish between opal from Andamooka, South Australia. They both have a similar hardness and contain the same play of color with a light background.
Brazilian Opal mainly comes from the Boi Morto mine. Boi Morto means "ox dead" in Portugese. This name came from an ox that fell off a cliff and died at the base of the opal occurrence during early opal prospecting of the area in the late 1900's. This Boi Morto Mine is around the area of Pedro Segundo (Pedro II), which is a small town in the state of Piaui', in Northeastern Brazil.
The Australian opal that I use, mostly known as Black precious opal, comes from Lightning Ridge in New South Wales, Australia. This type of natural opal has a very distinguishable look compared to other types of opals. This stone was given the name Black Opal due to the stones dark background color. It is like a colorful rainbow flashing light in front of a black night sky. Black Opal from Lightning Ridge is considered the rarest and most prized opal in the world. After working with this magnificent gem, I found that it is more stable than most types of natural opal.
I purchase these types of natural opals in small parcels of opal rough. Each opal nugget has clay coatings and potch (common opal) around the lines of fire. This needs to be either ground or cut to expose what is inside each piece of rough. Once the lines of fire are partially visible they can be graded and classified into certain categories such as cabochons or inlay pieces.
When finishing opal I notice that any type of contamination or minute mistakes will show up on the finished surface. I make sure that before I set an opal that the surface has a fine polish and that the back of the stone needs to be ground to the desired depth. It is common for other jewelers to mix dark dye with the epoxy to give the opal a better brilliance of color. I will use the same method but it depends entirely on the stone that I am setting. When I set a rare Black Opal into a deep channel there is no point in enhancing the color with dye. If you are interested in Natural Opal, then why not be interested in the finest opal in the world? | 
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