Glass microspheres containing a substantial percentage of spheres having two or more optically visible gas bubbles per sphere provide a high level of whitening to composite materials containing them. The volume of gas bubbles is between 8 and 35 percent of the volume of the spheres on average, and the spheres exhibit good resistance to breakage upon exposure to high pressures such as may be encountered in polymer extrusion processes. The spheres have a median particle diameter of between 5 and 100 .mu.m, and at least 10% by number of the spheres incorporate at least two gas bubbles.