By Don Skillman
There is little in lapidary as satisfying as beginning with a piece of opal rough (as it came from the ground) and using judgement and skill, turning it into a beautiful precious gemstone, flashing with color and just begging to be set into a jewelery piece.
First of all, if you’re a beginner at cutting opal, read a copy of OPAL CUTTING MADE EASY, by Paul Downing. This short, illustrated book takes the novice through all the basic steps, and the reasons for them. While it is written about cutting mainly Australian opal, most of the instruction holds true regardless of which major type of opal you decide to work with. The subject of cutting opal is much too involved to treat appropriately in a single article, so make your way through the book first, and then the following hints may make sense, and even help you to produce suitable gemstones.
Virgin Valley (VV), Nevada opal is much different from Australian opal. VV opal has a greater tendency to absorb water during the cutting process, which can lead to fracturing during cutting as internal stresses are introduced. High absorption also means great care is required when dopping (attatching the rough to a dowel to facilitate holding while cutting)because some liquids, such as super glue, are actually drawn into the structure of the stone, making release of the dop difficult. Also, hot wax dopping, common with many lapidaries, is inadvisable as heat will increase internal stresses. I often use Elmers carpenter glue, allowing it to dry overnight before beginning cutting.
Perhaps the most striking difference between Australian and VV opal is the way the fire, or color (called ‘play of color’ by pros) exists within the opal stone. Australian opal tends to have “layers” of color, arranged in parallel, like sheets of paper lying on a tabletop. Cutting to the best color then requires careful grinding away over the surface of the stone, until the best color layers are exposed over the largest surface. VV opal, on the other hand, usually has play of color distributed throughout much of the entire stone, and visible from more than one angle. This makes it easy for the beginner to cut a stone with some play of color. But it also creates a challenge for the experienced, to produce a gem of the best play of color.
Skill in orienting a stone for the best color can only come with practice. Several factors always enter into consideration at this stage: The size of the rough stone, the shape of the stone, how many pits, craters, or dips are on what you want to use for the face, and the final size/shape you wish to achieve. Generally, orienting the rough stone for the brightest color is worthwhile, unless too much size will be sacrificed. Opal, because it is a precious stone with great value, is cut “free form”, to allow the greatest carat weight to survive. This means that if you cut away good opal to reach a shape that conforms with a perfect oval, you have minimized the weight of a stone that would have been larger if cut into a pleasing, but not standard, shape. And that’s one key, pleasing shape.
Because color is composed of microscopic spheres, and because color is distributed throughout VV opal, it is easy to see that most of what you are grinding is composed of those same spheres. VV opal will melt away on your grinding wheel very easily, so do as Paul Downing advises; “Cut a little, look a lot.”
For initial shaping, avoid using a grinding wheel coarser than 220 grit, as doing so will introduce minute fractures into your stone. Use adequate water, as dry wheels will overheat the opal. I finish grind VV opal on a 14,000 diamond wheel, and then go, carefully, to a 50,000 diamond lap, used nearly dry but with diamond lubricant. Care must be taken here, as overheating happens quickly.
Other, obscure properties of VV opal will reveal themselves to you as you gain experience. But the above are the main differences. Start with your poorest specimens rather than your best. Before long, you will discover why VV opal, from the Bonanza Opal Mine, are “The brightest opals in the World”