My crystals often have hollows in the middle or at one end, especially
the larger ones. I was never able to figure out what the relevant
variable was - under what should have been exactly the same conditions
sometimes they were hollow and sometimes they weren't. (Even in the
same tray). My advice is simply Don't Panic - my most usefull
derivative data came from a crystal with a large hollow in the center.
Phoebe at vger.niddk.nih.gov