I have only used thicker cork from hot pads/trivets. Discarding or hiding the rounded(in my case) edges. You also want to seal the cork with super glue before painting it, bare cork will suck up all your paint and it would take layers and layers for you to get the color you want. When sealing with superglue, make sure you're in a vented area as you'll end up using more than your average amount and it may burn your eyes and shouldn't be inhaled.
They can be used on their own to represent small rocks or be used as plinths to raise the mini up some. Here are some examples, where I have used cork in my bases:
This:
Became this:
Here it is as small rocks:
And here again as a plinth to raise the mini up.
Hope this helps. .
I've heard of guys stacking thinner sheets of cork and that may be something worth looking into. I would just stack up a few sheets and then use a sanding sponge, file, and/or xacto knife to carve them into shapes.