As Josh says, I've seen actual cork (I think you can get it at home depot) used to good effect. However, I've had really good luck with dried pine bark which is sold as mulch. Just pop it in your oven to dry and then stack and paint.
This is what it looks like on a base pressed into some premixed concrete patch and no other basing material.
Not to get on too much of a tangent (you seem to be looking for terrain supplies) but for basing some folks use real slate. I haven't tried this yet but I happen to have a fair amount of the stuff that I just scavenged this week as my neighbor is putting on a new roof and the former owner just covered over the 100-year-old slate tiles with tar paper. You're welcome to some of that if you like.
Actually any of you folks are welcome to bark or slate. Just let me know what you need and you can pick it up at a meeting. I've got plenty for figure basing, but probably not enough for terrain.
I agree with Gattz that sometimes rock basing can be rather high. The figs below are glued directly to the base.
The leader is a slotta fig and is based as such, the others have cast-on bases. Rather than basing them even higher, I disguised the cast-on bases with pieces of bark. If you want a really impressive base but want to keep the height down, basing on a washer or using bases from proxie models (which are thinner than standard) can help.
http://www.proxiemodels.com/store2/inde ... ry&path=20One last thing I'll suggest is that foam rubber -when torn and painted with acrylic paint- can make really fast and durrable hills. Though they aren't quite as realistic as insulation foam, they are much more durrable and less brittle. They don't require a base and they squish rather than chip.
I made some for Mattias
https://www.chicagoskirmishwargames.com ... gotiation/Just tear/cut, paint black, paint base color, drybrush with lighter shade and done!