Mixing clay and crystal litter does not pose a health issue and is not considered harmful to cats. But it is advisable not to combine the two different types of litter because the crystals will absorb the urine quicker than the clay pebbles, which will waste the clay litter and make scooping the litter box more difficult.
Mixing the two types of litter will result in greater difficulty scooping the box because the clumping properties of the clay litter are lessened from the non-clumping litter. This will cause you to empty the box more often, which will add more waste to the landfills and require you to purchase cat litter more often.
Using both types of litter in the same box will result in cleaning the litter box and changing the litter more often, which will create additional unnecessary waste.
According to Pet MD, most crystal litters are manufactured from amorphous silica, which absorbs the urine and odors within seconds of contact with moistures. The amorphous silica is deemed non-toxic to cats and humans, which means that crystal litters do not pose health concerns for your cats.
Kittens will often eat small amounts of cat litter when using the litter box or learning to groom their paws. If enough litter crystals or pebbles are ingested, it will cause gastrointestinal issues or, in some cases, death.