Ethiopian Opal is cut and polished in a similar method to most other Opals from around the world. Learning how to cut Ethiopian Opal is as simple as cutting most other Opals but with one exception. The exception comes from the fact that Ethiopian Opal is Hyrdophane, that is, it absorbs water. So more importance must be made in selecting and sorting the Ethiopian rough.
All Ethiopian Opal Rough should be fracture tested before cutting and polishing or you could end up re-cutting a stone with crack in it.
One popular method is to use blunt metal object like a screwdriver with a blunt end and use a rubber mallet to tap suspect areas. This will result in cleavage on the weakest point of the crack. You might end up with several pieces but they should be stable after this.
Another method is the water treatment method. By adding water from cool to warm water you can expose cracks and make sure you dry out the opal completely.
Note it has been common practice to fracture test Gemstones and Opals for centuries. All major carving centers of the world fracture tested rocks before carving as it is and was very time consuming to cut a stone so they wanted to be sure they had a stable one.
Some people suggest cutting Ethiopian Opal dry but we get better results cutting with water.
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Hi Queen of Gems, and Monty,
Great article on Et, Welo cutting. Fracture testing is a long tested aid in exposing fractures along with light refraction from suspect areas.
Refracting light with sunlight, flashlight and laser are all very useful. One of the most useful tools I use was touched on in the very informative tutorial, is the emersion test with hydrophane opal. Very simply pace the rough, one or more stones, in lukewarm water for just a short time, a minute or so and if the rough starts to show a clear line, or even several clear lines, the more time in the water the more visible the clear lines become. Those clear lines are fractures that are allowing water to penetrate the interior just as it does the surface of the stones start to clear.
At that time I take a sharp pencil and trace those clear lines putting a path to see after the stone dries back out and I use these traces to then part those cracks. The first thing I try is just breaking them with my hands. If some part but some don't the next thing is a sharp Exacto type knife in the seam, and the last tool is a soft tap with the blunt tool as described in the article.
The sharp tool most often splits the fracture and it continues, where it hadn't yet fractured, cleanly through where the natural fracture line is going to run. The blunt tool has a tendency to start more small fractures at the point of impact which may, or may not, cause additional problems in the opal. This isn't usually the case in other stones such as quartz or others.
Monty, sorry to take so long in getting to this, but in my opal history (60 years since 1956), any opal that sticks together and has pretty color has value, which varies from person to person. Your gem of 8 years with the fracture "fire" in the back of the gem has proven it isn't going to cause any more fracture problems, short of leaving it in the hot sun, and you should enjoy it. I even see many gems with obvious fractures in auctions and for sale, so there is a market for beauty with disregard for perfection!!
As always, the Opal Auction family and lapidary as a whole is eager to widely share their knowledge as shown by Queen of Gems and in this fine article!!
Monty it sounds like you have been having fun cutting Ethiopian opal for 8 years?
Originally a lot of Ethiopian opals had cracking problem but Welo opals are more stable now compared to 10 years ago when a lot of surface opals were mined as they were easy to obtain. Miners now dig deeper and seems the production is a lot better.
A lot of opal cutters now are doing ”fracture” testing on large amount of rough opals before cutting.
Quartz and mineral crystal cutters have always “fracture” tested before cutting and I was once in a factory in China that was carving quartz crystal skulls, and they poured boiling water and then freezing cold water on crystal to see if any cracks appeared.
You can imagine there is a lot of work to carve a skull so they did not want cracks to appear while carving them.
Most Ethiopian cutters do this fracture test now before cutting the rough opal and if there is fracture it is easy to spot.
If opal has fissure lines or crack lines inside I think they are still valuable, but if there is a crack on the surface, the opal is considerably less but not worthless.
I use strong torch to see if line reflects light and if it does, I consider it a crack, but many lines do not reflect light so I consider “fissure” testing on the opals are still valuable.
“One man’s trash is another man’s treasure’’ as the saying goes.
This comment was in response to the article by Chuck Starbrid
1. I enjoyed your article on the opal auction web site. I got a bad taste on the older Ethiopian opals. I did get one of those to cut without cracks, I left the host rock backing on. The opal is now about 8 year
2. I recently started trying to cut the welo opals. I have fair luck with the small ones. But with the large ones it is poor luck. The best one I cut so far was clean looking at it on the surface but a light shined from the bottom up showed plenty of fissure’s. Cracked opal are considered valueless, would that opal be consider valueless. Any help for me.