Pebble painting is one of my favourite activity. It's always easy to find some pebbles with interesting shapes, although round, flat and symmetric pebbles are very rare here. They are generally from riversides, like Danube or Tisza. Their material can be limestone, some volcanic like basalt or quartz, which is the hardest and the most common. Well, I prefer limestone because it can be polished and colored easily, but needs some surface protection, like water-based lacquer.
These photos are original and taken with Samsung Galaxy GrandNeoPlus, edited in RawTherapee.