What I would cut if I were cutting

kw: rocks, minerals, lapidary

It has been a couple decades since I did any stone cutting. During the few years that I was a lapidary hobbyist, I soon found that I got the most satisfaction from cutting cabochons from precious opal.

Common opal is a whitish variety of hydrated silica (quartz plus water) with a very smooth texture. Precious opal, or fire opal, blazes with any or all colors of the spectrum. The difference is internal structure.

All opal is composed of spheres of silica, usually smaller than a micron in size, with a combination of amorphous silica and water in the spaces between the spheres. Two characteristics make common opal "common". Firstly, the spheres are larger and have a range of sizes. By "larger" I mean large enough that, no matter how they are packed, they will not diffract light efficiently; this typically means larger than one micron in average diameter. Secondly, the spheres, particularly because they are not uniform in size, are packed in a rather amorphous of chaotic arrangement. For this reason, common opal is rather soft, typically about 5.5 on the Moh's scale.

If the spheres are more uniform in size, they are likely to pack well into a closest-packing arrangement (like a well-stacked pyramid of oranges at the market). Their average size will then determine how much of the spectrum they diffract into the blazing colors of fire opal. Larger spheres diffract mainly red and yellow light, making red or orange fire opal. Smaller spheres diffract more green and blue light, while still diffracting some of the longer (redder) wavelengths. The smallest spheres produce mainly blue-green and green appearing stones.

When I was cutting opals, I found that they varied in durability. Not so much due to varying hardness, but because of how well the water content was bound in the stone. I learned to ask the dealer how stable the material was. An honest dealer will tell you, and point out which stones have to be kept in water to retain their fire, and which can be cut into a cabochon that will last the longest. Of course, the latter will cost more. Like a lot of rockhounds, I wound up with a jar of water containing nice looking opal rough sitting on my mantel, stones I'll never be able to cut, but they do look pretty.

During those years, I got to know one man who didn't own any lapidary equipment, but was cutting fine stones that won awards at rock shows. He said he'd buy a likely-looking piece of rough and grind it on the sidewalk next to his front step. If the fiery layer was buried too deep in the "common" portion of the stone, he'd pay the dealer to cut some of it off first with a diamond saw. At home he would simply hold the stone by hand and grind it on the wet pavement. He had one section of the pavement that was well worn from years of grinding, which was now his "fine grind" section. A light touch helped also, to achieve a nearly smooth finish. When he was satisfied, he'd go back to the dealer and pay to have the stone polished on a rouge-loaded felt wheel. The cost wasn't much, because a good polish took just a few minutes to produce.

Even the hardest opal is no harder than Moh's 6.5, so it cuts a lot easier than agate or jasper. I found I could make a finished opal cabochon in a quarter of the time it took me to finish a jasper "cab". If I ever return to the lapidary hobby, rather than collecting just those pre-finished items called "mineral specimens", opal will be first on my list of materials to cut.