They are flower rays from Columbia. I still have the pair. They get darker with age and eventually they will turn black inbetween the flowering in most cases
I went back to sand to give it a more natural look plus I wanted to have a lighter substrate to make my marbles lighten up. Bare bottoms and tile bottoms are easier to keep clean but do not do much as far as show. I have one sand tank and one bare bottom but think I am going to convert the bare bottom to sand as well