I have always thought "long" rocks were particularly cool.
It's pretty rare to find igneous rocks (granite, basalt, etc) in long shapes, although there are exceptions. Most "long" rocks I find tend to be sedimentary in nature-- limestone, siltstone, sandstone, shale, etc.
No matter how you look at it, truly "long" rocks (like the one in the top of the photo) are quite rare. However, I really like working with this shape because it lends itself so well to having a "string" or "snake" design added. The dark stone in the center, with the white design, is a rare piece of "long" basalt I found, earlier this spring.
The two other (primary) shapes of rocks I like to paint on are flat rounds and thick oval (egg) shapes.