Not to add to the confusion but rather to help explain it a little more....
Lots of times you will see folks "sculpting" glass on the streets making various items.
Roses, wizards, sun catcher's, etc.
Typically EVERY time they are using borosilicate or pyrex glass, which as Scott explained fairly well is a specific COE.
This glass, (the same stuff you use in your kitchen pans) is VERY hard and can take a LOT of stress.
That is why you can see many of the glass artist make a glass item and leave it to cool down on their graphite slab that they work on.
It is NOT annealled, however, normally if the temperature is not too cold, it will be ok and will never show fractures because it has cooled down slowely enough to not stress the glass to the point that it will break.
One of the main reasons is the thickness being made. It is normally not that thick and can cool down rather evenly.
However, a marble on the other hand is really thick in comparison, even a 3/4" size probably will fracture if not cooled down evenly and slowely
as Scott has previously said.
The compatibality issue especially with the aventurines and goldstone (Lutz) is that it is normally so soft that it is virtually impossible to anneal evenly.
Here again a main consideration being thickness as it is used sparingly (due to cost) and even if just on the surface will cause too much stress in the glass for some fracturing, or "checking" of the glass not to occur.
It can be said that it is the effect the artist was looking for. I would have to say that is what was decided to say after the fact when the fracturing or checking showed up and they did not want to throw it away due to losing too much money.
As I am not a glass artist, but a hobbyist I get to play with the glass. I do not have to make glass that will sell.
I get to make LOTS of tests and mistakes and like Scott I catalog them so when we do start to make marbles we want to sell, we know what glass we can use, normally how thick we can get away with its use, even if we can use it on the surface and not the interior or vice versa.
Fractures are BAD glass.
If you pay for "broken" glass, (except crackle glass, which is A highly refined technique) you are buying someones mistakes.
Either bad compability or not enough annealling. Regardless, it is still bad glass.
Bo