You can get botryoidal geodes, which will have smooth formations inside, instead of sharp quartz crystals. Several types of rock form these types of geodes, such as agates and some rare quartz. You could also go with drusy (lots and lots of little tiny crystals amassed inside the geode), which tend to be MUCH less sharp. As theshadybird was saying, test anything you might put into your tank for a few weeks prior to actually using it. I would also recommend giving any cut rock you're going to put into your tank a REALLY good scrubbing with white vinegar and hot water and rinsing thoroughly, as some of the polishing compounds contain high levels of iron, aluminum or other things that could contaminate your tank.