Depending where they come from may not be too rare.
Many years ago someone asked me to facet some garnet from Gemtree NT. These were chunks of perhaps 25-50ct., a beautiful deep red and seemingly few flaws.
The problem was that this material (perhaps due to its cooling as the crystals formed) had very high internal stresses which resulted in the stones shattering as they were facetted.
This also happed with zircon from the same area, I had 4 cut stones ready to mount on a brooch when one suddenly went "poof" into a million fragments!
I hope you dont experience this with yours, as I mention it may well depend where they came from.
Mike