Natuaral slabs are nice, I collect a few and natural bowl pots from time2 time. The weight is a major draw back though, when you get into the good sized stuff like over 3' long they require extra hands for lifting. Drilling holes for tieing material down or providing drainage is a common practice for stone slabs, also cutting, chiseling or grinding them into shape is done. You may also look at marble slabs, material can be planted on these or also used the same way one would a bamboo mat to dislay a suiseki or accent plant. I worked as a tile setter for about 6 years, I really wish I had been into trees then, throwing away marble tiles etc.. that could have easily had the edges rounded off and shaped for display slabs was common enough.
Here is a sample of some I have collected.
http://pictures.bonsaitalk.com/show...&cat=500&page=1
http://pictures.bonsaitalk.com/show...&cat=500&page=2