I have always thought "long" rocks were particularly cool.
For the most part, they are also pretty uncommon, mostly originating from "holes" in other kinds of rocks that filled with sediment of some kind, which then hardened into a rock... and was perhaps "released" when the other rocky material around it eroded away.
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.