A valve for controlling a flow of abrasive particles suspended in a pressurized carrier fluid has at least two apertured valve seats (74, 75) in face to face contact. One of the valve seats (74, 75) may be slid between a first position in which the apertures of each valve seat (74, 75) are aligned to allow fluid flow and a second position in which the aperture is one valve seat (74, 75) is blocked by the face of another (75, 74) to stop flow through the valve. The valve seats (74, 75) each have an outer layer of material with a hardness on the Mohs scale of at least 9, such as diamond. The valve is suitable for use in a fluid jet machining apparatus, particularly apparatus charged with a suspension of abrasive particles such as garnet in water.