::SPHERE FACTOR::

The world's most perfect man-made spheres have been produced, for use in a gyroscope for Gravity Probe B. "Topped in sphericity only by neutron stars," they're made of fused and polished quartz coated in a thin layer of niobium. They're about the size of a ping-pong ball,and their tolerances are astounding:

"On its surface, the gyroscope is less than 3 ten-millionths of an inch from perfect sphericity. This means that every point on the surface of the gyroscope is the exact same distance from the center of the gyroscope to within 0.0000003 inches."

Link. And another link.

(via Astronomy Picture of the Day)