These are some references for cutting opal. Often, opal – especially precious opal – is present as thin bands or veins within the rock. Boulder opal – such as koroit boulder opal, from Australia, typically forms veins within an ironstone. The ironstone and the opal have very different hardnesses. You may choose to expose more of the opal, or cut the stone so the opal forms as patterns of veins. With Koroit boulder opal, the patterns in the ironstone often are as dramatic as the veins of the opal and help to enrich the opal. Opal can be cut with tools as basic as a dremel and do not necessarily need all the full lapidary equipment.
Samples of finihed pieces of Koroit boulder opal. Courtesy of S.K. (Australia). 1 – sample of a piece cut for inlay, 2 – four pieces of boulder opal cut and formed into a cube and then put on a pendant, 3 – a finished inlay, 4 – four small cores taken from a boulder opal and put together to make a pendant.
CUTTING PRECIOUS OPALS
Idaho Spencer Opal is a type of precious opal with color bars within common opal. For this material, you will generally cut right along the color bar to expose it, then polish it down. Due to the thinness of a color bar and the fragile nature of opal, opals are often backed (doublets). Doublets are not worth a much as a pure opal, but they look just as good. Triplets can also be made with a back and a thin piece of quartz glass (which can be obtained as “scrap” from a jewelry shop, used for watches).
The key with seam opal like Spencer opal is to expose the color band, which can be thin, without cutting through it. Below is a sample of Spencer Opal showing the color bands.
The goal would be to cut the opal right above the color band, either exposing just the top of the color band, or leaving a thin layer of the white common opal on top of it. Then cut below the color band. If the opal below is thick enough and strong enough, you may be able to shape it without making a doublet. If it is fragile or thin, you will need to make a doublet. A doublet is simply adding a thin backing of stone such as basalt (basanite). The backing can serve multiple purposes. First, it can strengthen the opal. Second, especially if you have “jelly opal” – which is translucent – it can really intensify the color of the opal.
Note: I would wet grind the backs to no finer than 220 grit (cuts down on the dust, no need to worry about silicosis). I recommend E3300 water clear epoxy to put the pieces together and would not necessarily color it with the black oxide.
A triplet, as the name implies, has three layers. On top of the opal is a thin layer of silica glass (jeweler’ glass, used for making watches). Assemble the three pieces the same as with a double. Be extra certain that you do not get bubbles in the epoxy. The top layer of glass, when it is cut into the shape of a cabochon, acts as a magnifier for the opal.
Pricing: Note that if you d anything to a stone, whether it is stabilizing, backing, filling cracks or pockets, etc, it must be disclosed. A solid opal is the highest priced opal. A doublet can reduce the price as much a half or more. A triplet will reduce it another half or more. However, these techniques are perfectly acceptable and can make otherwise unusable material into gorgeous pieces.
Doublets can have other unusual uses as well, that is a topic for another post.
Bacon opal (bubble opal, candy stripe opal) from Utah is interesting because it can be cut and slabbed as a matrix opal, or it can be carved out and the opal exposed as a bubbly, intricate surface that make for interesting (and pricy!) cabochons. The opal by itself is called hyalite opal. In one place in Utah, they are now investigating this material for use as a whitener in tooth paste!
Much of the work with opals can be done with a dremel. In fact, for fine shaping and following the opal bars, the dremel is often preferable to a cabbing machine. Mini smoothing discs and disc holders are available from Hi-Tech Diamond. As a partner with Hi-Tech, if you use my links and code janet at ceckout, you will receive 20% off.