So it wasn't exactly love at first site when I discovered these snake-looking stones in an old collection of rocks we bought.   But the material has grown on me.  (Not the snakes though!)   It's a variety of chalcedony with this pattern that looks like the skin of a snake.   The material can be found in Oregon and Idaho among other places.

You can tumble this material and if done right, you preserve the 'snake-look' of the pieces.  We have never tumbled any of it ourselves, but we ran into someone at Quartzsite  last year that had (Quartzsite, Arizona - large rock show there every year).   As I recall, he said you needed to tumble for short times and check it often so you don't tumble away the pattern.   A quick look online and some appear to have lost some of the pattering in the tumbling, but you can find other pieces that look really amazing!   

So if you know of a rockhound who also likes snakes, the scaly look of snakeskin agate makes a pretty cool piece, whether as a tumble-polished stone or even in it's natural form.