Amethyst and some quartz can be a problem as others have already mentioned,,,,A lot depends on the rough itself.If it's fractured to begin with it needs a little TLC when doing it.
I did about 6-8 lbs of chevron amethyst a while back for a friend to use as place settings at his wedding dinner.
I did a lot of "window shopping" on the web to try to find the best quality amethyst rough for my money.
Although just about all of the rough is fractured somewhat I did find some that wasn't as bad as the others,,,,,
This is the rough shown wet,,,,
I roughed it for 10 days with 60/90 and no plastic pellets.
I had a little wear after the 10 days so I added plastic pellets to make up the volume loss and also to cushion the amethyst.
As the roughing continued I took out the pieces that were ready for the 2nd stage and added more pellets for volume loss.
After several roughings I finished the amethyst in my Lot-O-Tumbler ,which is a vibe unit.
If you look closely in these photos you will see internal fractures which were there when the amethyst was formed in the earth but you won't see any fractures that were caused during the tumbling process,,,,,,You will also see "water marks" on the dry amethyst ,,,
I took this photo during or after the rough stage to show that even very small pieces can be done without being fractured,,,,
Two pieces sitting on a Bic Lighter,,,
I seldom use plastic pellets when roughing rocks but some rocks need that cushion or the whole load gets fractured.
I have found that the easiest way (for me) to "cope" with pellets is that after dumping the load into a calendar and rinsing it off good with a hose,,,or several containers of water,,,I then dump the whole load(rocks and pellets) into a container full of water.
The pellets float to the top and I just scoop them out with a tea strainer
If n fact you are doing something like amethyst or other delicate material(s) you don't have to separate the pellets from the rocks until your final rough.
I'm not all that fond of using pellets but sometime they are necessary,,,,
I don't like a snotty tissue either but if I have to blow my nose,,,,,, ;D
connrock