Loofah washing up sponge
My grandmother used to grow a loofah plant in her garden and the fruit looked very similar to cucumbers. She would then dry them in the sun and finally use the fruit skeleton to wash her dishes. As a child I always remember a section of the loofah soaking in a bowl of soapy water by the kitchen sink which she would use for washing up.
So if you are looking for a 100% natural washing up "sponge" which is fully compostable and does not damage the environment (as harvesting natural sponges does) then look no further.
No nasty plastic sponges to litter our environment. Otherwise go a step further and plant your own loofah plant.