A downhole ball valve includes a housing that includes a tubular member; a ball seat positioned in the tubular member, the ball seat including a sealing surface; and a ball that includes a first hemispherical portion, a second hemispherical portion, and a bore that extends through the ball between the first and second hemispherical portions. The ball is adjustable between a closed position with the first hemispherical portion sealingly engaged with the sealing surface of the ball seat to close the bore to fluid communication with the tubular member, and an open position with the bore at least partially in fluid communication with the tubular member. The first hemispherical portion includes a first material, and at least a portion of the second hemispherical portion that extends from a surface of the ball towards the bore of the ball includes a second material different than the first material.