The gravel that we most frequently recommend for use as the substrate in the trays of the Pet Patio Potty™ is pea gravel or pea pebbles. This type of stone may also be called 3/8” river rock. This type of gravel is very inexpensive and can be acquired at most home supply stores such as Home Depot or at landscape supply companies. For larger dogs, the pea size gravel may get caught in their pads so a larger size stone should be considered, for example, 3/4” river rock. Any type of decorative stone with sharp edges should be avoided. Also, pebbles smaller than 3/8” diameter should not be used because they will fall through the 1/4” perforations in the polyethylene tray bottoms. Please note that absorbent litter and gravel are not included with orders.