Gang,
I just got back from a long weekend in Vegas. I was there with family in tow, but when it came time to gamble, while they were content at slot machines, I went and played poker (I did OK breaking even after about 12 hours total).
I spent most of my time at the MGM Grand and it was when I was cashing out on my last evening that I saw a horrific thing. There were two people at the cashier cage, one of whom cashed me out and another with a damp terrycloth rag wiping down chips!!

While I suspect this is probably why the chips didn't look all nasty, it would explain the rounded edges on many of the chips and the lack of surface texture on others.
I'm pretty new to this whole "chip thing", but common sense says scrubbing on a clay chip with a wet cloth is just asking to scrub away a lot of the chip.
While there are a lot of really classy chips out there, the MGM is probably my favorite. Simple and bold in the inlay design. I managed to pick and choose out of my stack a few essentially brand new $1 chips and the cashier that helped me that last night was kind enough to sift through what she had in her drawer to find an almost pristine $5 for my own collection. I couldn't manage to get one of the $3 rake chips they use though (they're really nice from what I could tell sitting there in the dealer tray).

Oh well.
Long story short, be gentle to your chips when cleaning them. Scrubbing with a damp rag is probably
not the way to go.
EDIT: I tweaked the images in Photoshop and I think they represent the real colors and appearance as opposed to the earlier post.