These coasters are made out of neoprene, and are about 4 inches in diameter. It's the same material mousepads are made of.
This makes them exceptionally good as coasters - they absorb really well. You can also clean them in a garment bag in your wash if you need to. The back is rubber.
The image on them are rainbows done in watercolour. The design was painted by hand, and then printed onto the coasters.
This is for a set of four. Each rainbow is a little different.