I'm not sure what "pearls" are, but I have used the crystals before. Tidy Cat crystals are good for a month if you only have one cat. Since I have 2 cats, the litter box was changed more often. As the box is used, the crystals change from clear to whitish-yellow (urine absorption) and you know when to change the box. Any moisture from feces is dried out, so when you scoop the poop, it's almost rock hard.
There are a few brands of crystals out there, but I prefered Tidy Cats. Some brands have really large crystals, which can hurt little paws, and the Fresh Step brand actually dyed my litterbox blue, so I never used that one again. If it can dye the box, than the cats are getting dye on their paws and that can't be good.
Mine didn't seem to have any problems with Tidy Cat, but because it is a tad more on the expensive side, I changed recently to a pine product called Feline Fresh.