Re: Corundum - How to know whether to cut/polish
If you can't see light through the center of the illuminated rough when you attempt to look through it, the prospects for it as a single, large faceted gemstone are dismal. It sounds to me like you have a mineral specimen, not cutting rough.
A mitigating factor could be an opaque rind or coating surrounding the material that would cut away during the faceting process. A more revealing test would be to grind through the surface of the stone on opposite sides to remove any such coating and then test its transparency again.
A common mistake made by beginning faceters is to check the color and overall transparency of the stone by shining a flashlight through it, or holding it up to a light source and attempting to look through it. While this is a good method for inspecting a stone for inclusions and other internal flaws, unless you plan to wear the stone with a flashlight under it this is not a good way to test whether it is too dark to facet. A large pyrope garnet can look a gorgeous blood red when you do this, but will look black once cut and set without an energizer bunny providing the extra photons.
Here is the gospel truth about whether your rough is too dark to make a satisfactory faceted stone: After you have cut and semi-polished faces on opposite sides of the stone, set it on top of a page of the Bible. If you can't make out the text through it, that is God telling you the stone is still too large to facet relative to its transparency.
The much abused term "facet grade" really doesn't mean much. You can facet driveway gravel if you are so inclined, as diamond laps will cut just about anything... A large stone like that unholy pyrope that is too dark to cut as a single stone can sometimes be satisfactorily faceted by cutting into a number of smaller stones. But you don't sound inclined to do that and if the stone is very dark you will have to cut very small stones from it for them to work satisfactorily as conventional faceted stones.
You really don't need an expert at the Smithsonian to tell you if your stone is a cutter or not. Any competent amateur faceter can provide a good prognosis. As mentioned above that may require some divine revelation, but something to keep in mind is once you grind faces on a stone to test for transparency you have butchered it as a mineral specimen.
Hope this helps some, even if it isn't particularly optimistic regarding your prospects of having found a cutter. Keep on rock'n!
From Bob Keller - August 20, 2007 at 09:01:01