Abstract: A non-contaminating gas-tight valve for controlling a flow of granular polysilicon. The valve has a spherical valve member formed from single-crystal polysilicon, so that any particles worn from the valve member during use will be non-contaminating polysilicon. The valve member has a passage through which granular polysilicon flows when the valve is in an open position. When rotated perpendicular to the flow, the passage no longer permits movement of granular polysilicon through the valve. The valve member has a smooth finish and is wiped clean when rotated against non-abrasive upper and lower seats, reducing the likelihood of valve member wear. A cavity between the valve member and the valve body allows for removal of excess granular polysilicon from the valve, inhibiting the valve from seizing due to excess granular polysilicon slipping past the upper valve seat and accumulating within the valve. The valve additionally forms a gas-tight seal between an upstream and downstream side of the valve.