Proppant
Proppants are the solid materials used in 'Hydraulic Fracturing'. In hydraulic fracturing, the cracks are opened by pumping fluid into the well with high surface pressure. Since the cracks are opened by force, they tend to close as soon as pressure is removed. Proppants are the solid particles that enter into the crack and don't allow the cracks to close. Materials like sand, bauxite, ceramic and some lighter weight materials are usually used as proppants.