What is that limescale type deposit in the bottom of the cat litter tray?

[CherryPeach675]

No matter how hard I try & clean it it doesn't come off. Even limescale remover doesn't remove it. What is that stuff?

Could be a type of corrosion from the wee. I had the same thing happen in my son's hamster tray..always in the spot where he weed.