Sand is the common material. Ceramic grains are used where the closure stress would crush sand and collapse the fracture conductivity. Mesh size is a trade: larger grains conduct more when they survive, smaller grains travel farther into a narrow fracture and pass a tighter perforation.
The pack only works where it was placed. Proppant that settles in the wellbore, or that is not carried to the far end of the fracture, leaves a fracture that closes there. Conductivity also falls if the grains crush or if polymer from the fracturing fluid stays in the pack.